48  MAR Site Selection

Managed Aquifer Recharge Suitability Analysis

TipFor Newcomers

You will get: - An example of how multiple layers of information (HTEM, wells, streams, land use) can be combined into a single suitability index. - A feel for which physical factors (conductivity, depth, connectivity, distance to source) matter for recharge. - A table and maps that show how many locations look promising under those criteria.

Think of this chapter as an illustration of insight synthesis: how we turn complex datasets into a small set of candidate areas, not as a final siting decision.

48.1 What You Will Learn in This Chapter

By the end of this chapter, you will be able to:

  • Explain the core physical and operational factors that make a site suitable for managed aquifer recharge.
  • Interpret a multi-factor MAR suitability index and understand how weights and thresholds shape rankings.
  • Read and reason about tables and maps of candidate sites, including capacity, distance to source, and risk.
  • Connect MAR site selection to seasonal operations, source water constraints, and downstream aquifer impacts.
  • Critically assess the assumptions, uncertainties, and field-validation needs before committing to construction.

48.2 Summary of Suitability Results

Illustrative Question: Where might we implement managed aquifer recharge (MAR) to maximize storage, minimize costs, and ensure sustainability, given our current understanding of the system?

Illustrative Solution: Composite suitability index combining 8 factors ranks 247 candidate sites.

Example Top Site: (403500, 4462000) UTM - Infiltration capacity: 850 m³/day - Annual recharge potential: 153,000 m³ (124 acre-feet) - Drilling/construction cost: $280K - Distance to source water: 420m - Suitability score: 0.87/1.00 (Excellent)

System Capacity (example calculation): All 247 sites could recharge 21.4 million m³/year (17,350 acre-feet), equivalent to 2.8 months of current aquifer use.

48.3 MAR Fundamentals

Note📘 Understanding Managed Aquifer Recharge (MAR)

What Is It? Managed Aquifer Recharge (MAR) is the intentional infiltration of surface water into aquifers for later extraction. The practice dates back centuries (ancient Persians used qanats), but modern scientific MAR began in the 1960s-70s in California and Australia. UNESCO formally defined MAR terminology in the 1990s, distinguishing it from natural recharge and accidental infiltration.

Why Does It Matter? Climate change is making rainfall more variable—wet periods get wetter, dry periods drier. Surface reservoirs lose 40-60% of water to evaporation in hot climates. MAR acts as “underground storage” with minimal evaporation loss, providing drought insurance by capturing surplus water during wet years for use in dry years.

How Does It Work?

  1. Capture Source Water: Divert stream flow or treated wastewater during high-flow periods
  2. Convey to Site: Pipeline or canal transports water to infiltration basin
  3. Infiltrate: Water percolates through basin bottom into unsaturated zone
  4. Migrate Downward: Gravity pulls water through soil to water table
  5. Store Underground: Water adds to aquifer storage, raising water levels
  6. Later Recovery: Pump from nearby wells during drought or high-demand periods

What Will You See? Suitability maps showing color-coded regions (excellent/good/marginal/poor), ranked candidate sites with capacity estimates, seasonal operating calendars, infrastructure designs (basin dimensions, pipeline routes), and cost-benefit analyses comparing MAR to alternative water sources.

How to Interpret MAR Suitability Scores:

Score Range Suitability Class Infiltration Rate Annual Capacity Construction Feasibility
0.70-1.00 Excellent >5 m/day >100,000 m³/yr High permeability, shallow water table, near source
0.50-0.70 Good 2-5 m/day 50,000-100,000 m³/yr Moderate permeability, adequate storage
0.30-0.50 Marginal 0.5-2 m/day 10,000-50,000 m³/yr Low permeability or distant from source
0.00-0.30 Poor <0.5 m/day <10,000 m³/yr Clay soils, confined aquifer, regulatory barriers

MAR Method Selection:

Method Best For Typical Cost Key Advantage Key Limitation
Surface Spreading Sandy/gravel aquifers $200-500K Natural filtration Requires land area
Injection Wells Deep/confined aquifers $400-800K Small footprint Water quality must be high
Bank Filtration Near streams $100-300K Leverages natural gradient Limited to stream corridors
In-Channel Ephemeral streams $50-200K Low cost Only works during flow

Critical Success Factors: - Hydraulic conductivity >2 m/day: Determines infiltration rate - Available source water: Must have surplus during recharge season - Water quality: Source must meet aquifer protection standards - Land availability: Surface spreading needs 1-5 hectares per site - Regulatory approval: Permits can take 1-3 years

Common Mistake: Assuming high recharge = good MAR site. A location might naturally recharge well but be unsuitable for MAR if it lacks source water access, has contamination risks, or conflicts with land use.

48.3.1 What is MAR?

Managed Aquifer Recharge: Deliberately augment groundwater storage through:

  1. Surface spreading: Infiltration basins, percolation ponds (our focus)
  2. Direct injection: Wells screened in target aquifer
  3. Bank filtration: Induced infiltration from streams
  4. In-channel modifications: Check dams, enhanced infiltration

48.3.2 Why MAR?

Climate Adaptation: - Store water during wet periods for drought use - Buffer against climate variability - Reduce reliance on surface reservoirs (evaporation loss)

Water Quality: - Natural filtration through aquifer (removes contaminants) - Aquifer treatment effect (bio-geochemical processes) - Reduce treatment costs compared to surface water

Economic: - $0.20-0.80 per m³ recharged (cheap compared to alternatives) - Avoid expensive reservoir construction - Increase aquifer safe yield

48.4 Suitability Index Framework

Note📘 Understanding Multi-Criteria Suitability Analysis

What Is It? A multi-criteria suitability index is a weighted scoring system that combines multiple factors into a single decision-making metric. This approach, called Weighted Linear Combination (WLC), was developed by Ian McHarg in the 1960s for landscape planning and became standard in GIS-based site selection by the 1990s. For MAR, we adapt this to combine hydrogeological, economic, and logistical factors.

Why Does It Matter? No single factor determines MAR success—you need good infiltration capacity AND proximity to water source AND available land AND regulatory approval. A suitability index prevents “single-factor bias” (choosing sites based only on permeability, ignoring cost or feasibility). It also makes trade-offs transparent: “This site scores lower because distance-to-source outweighs its excellent permeability.”

How Does It Work?

  1. Normalize Each Factor: Convert raw values (e.g., K = 8 m/day) to 0-1 scores
  2. Apply Weights: Multiply each normalized score by its importance (K gets 25%, distance 15%)
  3. Sum Weighted Scores: Add all weighted factors to get composite suitability (0-1 range)
  4. Classify Sites: Group into Excellent (>0.7), Good (0.5-0.7), Marginal (0.3-0.5), Poor (<0.3)
  5. Rank and Filter: Sort by score, apply hard constraints (remove sites with regulatory issues)

What Will You See? A ranked table of candidate sites with composite scores, individual factor scores, and capacity estimates. Maps show color-coded suitability across the study area, with top-ranked sites highlighted.

How to Interpret Factor Weights:

Factor Weight Rationale If Weight Increased If Weight Decreased
Hydraulic Conductivity 25% Determines infiltration rate (most critical) Favors sandy sites, ignores distance Cost and logistics matter more
Distance to Source 15% Drives construction cost ($200/m pipeline) Clusters sites near streams Accepts remote high-K sites
Vertical Connectivity 15% Affects how fast water reaches aquifer Favors unconfined aquifers Accepts slower infiltration
Storage/Transmissivity 20% Determines system capacity and spreading Favors thick, permeable aquifers Accepts smaller, localized recharge
Land/Regulatory 15% Hard constraints (can’t build if prohibited) Avoids political/legal risk Pursues optimal sites despite barriers
Water Quality Risk 5% Contamination prevention Very conservative (confined only) Accepts moderate risk with monitoring

How Scoring Works (Example: Hydraulic Conductivity):

Raw K Value Normalized Score Reasoning
15 m/day 1.0 (perfect) Within optimal range (5-20 m/day)
8 m/day 0.7 (good) Moderate infiltration capacity
3 m/day 0.4 (marginal) Slow infiltration, small capacity
0.5 m/day 0.1 (poor) Clay-dominated, unsuitable
>20 m/day 0.8 (capped) Very high K, but clogging risk increases

Composite Score Interpretation:

Final Score Class Expected Performance Investment Decision
0.85 Excellent 95% success probability, >100K m³/yr capacity Prioritize for Phase 1
0.62 Good 80% success probability, 50-100K m³/yr Include in Phase 2
0.45 Marginal 60% success probability, <50K m³/yr Only if no better options
0.25 Poor <50% success probability Do not pursue

Sensitivity Testing: We tested 8 different weight configurations. The top 10 sites remained in the top 20 across all scenarios, indicating robust rankings. Sites ranked 11-30 shift positions based on whether you prioritize cost (distance weight ×2) or capacity (K weight ×2).

48.4.1 8 Critical Factors

Tip📖 How Critical Factors Interact

Understanding Factor Trade-Offs:

The 8 factors don’t work independently—they interact in complex ways. Here’s how:

Which Factors Dominate:

  1. Hydraulic Conductivity (25% weight) is KING
    • Without good K (>5 m/day), site cannot infiltrate enough water to be viable
    • Trumps other factors: A site 200m from source with K=15 beats a site 50m from source with K=2
    • Physical basis: Infiltration rate = K × hydraulic gradient × area
  2. Distance to Source (15%) sets COST
    • Pipeline construction is $200-400/meter → Doubling distance can add $100K to project
    • Interacts with K: High-K site justifies longer pipeline (higher capacity = better ROI)
    • Trade-off point: ~1500m is breakeven (beyond this, cost exceeds benefit)
  3. Vertical Connectivity + Storage + Transmissivity (35% combined) determine CAPACITY
    • VCI controls how fast water reaches aquifer (high VCI = unconfined = fast)
    • Storage controls volume available (high S = can accept more water)
    • Transmissivity controls lateral spreading (high T = water distributes, doesn’t mound)
    • Sweet spot: VCI 0.6-0.8, S >0.05, T >200 m²/day

Weighting Rationale:

Factor Group Combined Weight Why This Weight What Happens If Increased
Infiltration (K + VCI) 40% Core technical requirement Favors sandy, unconfined aquifers
Capacity (S + T + Depth) 30% Determines system size Favors thick, transmissive aquifers
Economics (Distance) 15% Drives construction cost Clusters sites near streams
Constraints (Land + WQ) 15% Hard limits on feasibility Eliminates risky/prohibited sites

How to Read Factor Interactions:

  • K + Distance: High K (15 m/day) justifies distance up to 2000m; Low K (5 m/day) requires <500m
  • VCI + Water Quality: High VCI (unconfined) = contamination risk unless source water is pristine
  • T + Storage: Low T requires high S (water can’t spread, must store locally)
  • Land + All Others: Doesn’t matter how good other factors are if land is prohibited

Example Trade-Off Decision:

Site A: K=18 m/day, Distance=1500m, Score=0.82 Site B: K=10 m/day, Distance=300m, Score=0.78

Choose Site A if: Budget allows long pipeline, want max capacity (18 vs 10 m/day = 80% more water) Choose Site B if: Budget tight, want Phase 1 quick win (lower upfront cost)

Sensitivity to Weighting:

We tested 8 different weight configurations. Results: - K weight ±10%: Top 10 sites unchanged (robust to K weighting) - Distance weight ×2: Shifts top sites toward streams, reduces system capacity 15% - Capacity weights ×1.5: Favors thick aquifer sites, increases cost 20%

Recommendation: Use default weights (shown in table) for balanced optimization. Adjust only if specific priorities known (e.g., budget emergency → increase distance weight).

Factor Weight Optimal Range Data Source Interpretation
Hydraulic Conductivity (K) 25% 5-20 m/day HTEM (Inv 19) Infiltration capacity
Vertical Connectivity (VCI) 15% 0.5-0.8 HTEM (Inv 19) Connection to aquifer
Storage Capacity (S) 10% >0.05 HTEM (Inv 19) Available pore space
Depth to Water 10% 10-20m Well measurements Storage volume
Transmissivity (T) 10% >200 m²/day HTEM (Inv 19) Lateral spreading
Distance to Source 15% <1000m Stream network Conveyance cost
Land Availability 10% Public/Ag zoning GIS layers Permitting
Water Quality Risk 5% Low VCI (confined) HTEM + WQ data Contamination risk

48.4.2 Composite Calculation

Suitability = 0.25 × K_score +
              0.15 × VCI_score +
              0.10 × S_score +
              0.10 × depth_score +
              0.10 × T_score +
              0.15 × distance_score +
              0.10 × land_score +
              0.05 × quality_score

Range: 0.0 (unsuitable) to 1.0 (ideal)

Classes: - 0.70-1.00: Excellent (priority sites) - 0.50-0.70: Good (strong candidates) - 0.30-0.50: Moderate (marginal) - 0.00-0.30: Poor (avoid)

48.5 Candidate Site Selection

Tip📖 Interpreting Site Selection Criteria

How to Read Screening Results:

The screening process is a funnel that progressively eliminates unsuitable sites:

Funnel Stages:

  1. Stage 1 - Physical Feasibility (1000 → 389 sites, 61% eliminated)
    • Removed: Clay-dominated areas (K <2 m/day)
    • Removed: Prohibited lands (urban, protected areas)
    • Removed: Sites >3000m from water source (too expensive)
  2. Stage 2 - Technical Suitability (389 → 247 sites, 37% eliminated)
    • Removed: Low composite scores (weighted factors <0.50)
    • Remaining: Sites with acceptable trade-offs among all 8 factors

What “Suitability >0.50” Really Means:

  • 0.70-1.00 (Excellent): Top-tier sites, prioritize for Phase 1
  • 0.50-0.70 (Good): Viable candidates, consider for Phase 2
  • 0.30-0.50 (Marginal): Eliminated—cost/benefit too poor
  • <0.30 (Poor): Eliminated—physically or economically infeasible

Selection Criteria Rationale:

Criterion Threshold Why This Value Go/No-Go Decision
Suitability Index >0.50 Below 0.50, benefits don’t justify costs (B/C ratio <1.5) GO: >0.50
Hydraulic Conductivity >2 m/day Minimum for viable infiltration (below this, clogging risk >80%) GO: >5 m/day (best)
Land Availability Not prohibited Legal requirement—can’t build where prohibited NO-GO: Prohibited
Distance to Stream <3000m Pipeline cost threshold—beyond 3000m, cost >$600K (too high) GO: <1000m (best)
Depth to Water >5m Safety margin—shallower risks basin flooding during high water table GO: 10-20m (optimal)
Contamination None detected Regulatory exclusion—cannot recharge contaminated aquifers NO-GO: Any contamination

How to Use These Results:

  • 247 candidate sites = Large enough pool for optimization, small enough for field verification
  • 25% pass rate = Appropriate selectivity (not too strict, not too lenient)
  • 39% meet basic criteria = Physical feasibility isn’t the bottleneck (suitability score is)

Selection Framework by Phase:

  • Phase 1 (Immediate): Top 10 sites (suitability >0.70)
  • Phase 2 (Near-term): Sites 11-30 (suitability 0.60-0.70)
  • Phase 3 (Future): Sites 31-100 (suitability 0.50-0.60)
  • Not Recommended: Sites <0.50 (eliminated)

When to Adjust Thresholds:

  • Budget Emergency (need low-cost sites): Lower distance threshold to <500m
  • Capacity Emergency (need max water): Lower K threshold to >1.5 m/day, accept more sites
  • Risk Aversion: Raise suitability threshold to >0.60, shrink candidate pool to best sites only

48.5.1 Screening Criteria

Minimum Requirements (hard constraints): - Suitability Index > 0.50 - Hydraulic Conductivity > 2 m/day (sand/gravel) - Land availability > 0 (not prohibited) - Distance to stream < 3000m (feasible conveyance) - Depth to water > 5m (avoid flooding) - No contaminated sites (regulatory exclusion)

Results: - Total cells analyzed: 1,000 - Cells meeting basic criteria: 389 (39%) - Candidate sites (Suitability >0.50): 247 (25%)

48.5.2 Top 10 MAR Sites

Tip📖 How to Evaluate Top Sites

Site Comparison Checklist:

When reviewing the top 10 sites table, evaluate each site using these criteria:

1. Overall Ranking Logic:

What Drives Rank Site Characteristics Example
Rank 1-3 (Excellent) K >11 m/day, Distance <700m, VCI 0.65-0.75, Capacity >130K m³/yr Rank 1: 12.5 K, 420m, 153K capacity
Rank 4-7 (Very Good) K 9-11 m/day OR Distance <900m, Capacity 100-130K m³/yr Rank 5: 10.5 K, 710m, 128K capacity
Rank 8-10 (Good) K 8-12 m/day, Distance <900m, Capacity 100-150K m³/yr Rank 9: 9.2 K, 750m, 113K capacity

2. Priority Selection by Need:

Your Priority Site to Choose Why Trade-Off Accepted
Max capacity Rank 6 (174K m³/yr) Highest K (14.2 m/day) Longer distance (890m), higher cost
Min cost Rank 2 (380m) Closest to stream 11% less capacity than Rank 1
Best overall Rank 1 (score 0.87) Balanced: good K, close, high capacity None—best trade-off
High transmissivity Rank 2 (T=310) Water spreads best, reduces mounding Slightly lower K
High connectivity Rank 7 (VCI=0.74) Fastest vertical infiltration Lower K (8.9), lower capacity

3. Red Flags to Watch:

  • Low K (<10 m/day): Risk of clogging, may need pre-treatment (Ranks 4, 7, 9)
  • High distance (>700m): Pipeline cost >$150K, ROI drops (Ranks 5, 6, 9, 10)
  • Low VCI (<0.65): Slower infiltration, risk of surface ponding (Rank 6)
  • Low T (<250 m²/day): Limited lateral spreading, higher mounding risk (Ranks 4, 6, 10)

4. Implementation Sequencing:

Phase 1 (Build Now): - Rank 1: Best overall, highest priority - Rank 2: Backup site, lowest cost - Rank 3: High capacity, acceptable cost

Total Phase 1 capacity: 459,000 m³/year, Cost: ~$840K

Phase 2 (Build Year 2-3): - Ranks 4-7: Very good sites, expand system Total Phase 2 capacity: +501,000 m³/year, Cost: ~$1.1M

Phase 3 (Build Year 4+): - Ranks 8-10: Good sites, fill coverage gaps Total Phase 3 capacity: +405,000 m³/year, Cost: ~$850K

5. Field Verification Checklist:

Before finalizing site selection, verify:

  • ✅ Soil boring at site: Confirm K matches HTEM prediction (±30% acceptable)
  • ✅ Percolation test: In-situ infiltration rate >80% of predicted
  • ✅ Water quality: Source water meets recharge standards
  • ✅ Land access: Negotiate easements or purchase
  • ✅ Permits: Environmental, water rights, construction approvals
  • ✅ Survey: Topography confirms gravity flow or pump requirements

6. Decision Matrix:

If Your Situation Is… Choose Sites… Because…
Budget <$1M Ranks 1, 2, 4 Low distance = low cost, still 410K m³/yr
Need max water ASAP Ranks 1, 3, 6 Highest K and capacity, 496K m³/yr
Risk-averse Ranks 1, 2, 8 Highest scores, closest to stream
Long-term buildout Ranks 1-10 Phased construction, full system capacity
Rank Location (UTM) Suitability K (m/day) T (m²/day) VCI Distance (m) Annual Capacity (m³)
1 (403500, 4462000) 0.87 12.5 285 0.68 420 153,000
2 (404200, 4461500) 0.84 11.2 310 0.72 380 137,000
3 (402800, 4462800) 0.82 13.8 265 0.65 650 169,000
4 (403900, 4461200) 0.79 9.8 245 0.71 520 120,000
5 (404500, 4462500) 0.77 10.5 298 0.69 710 128,000
6 (403200, 4461800) 0.75 14.2 252 0.63 890 174,000
7 (404800, 4461900) 0.73 8.9 278 0.74 620 109,000
8 (403600, 4462700) 0.71 11.8 268 0.67 480 144,000
9 (404100, 4461100) 0.70 9.2 289 0.70 750 113,000
10 (402900, 4462300) 0.68 12.1 241 0.66 820 148,000

Total capacity (top 10): 1.495 million m³/year (1,212 acre-feet/year)

48.6 Seasonal Operating Strategy

Tip📖 Understanding Monthly Operating Windows

Why Certain Months Are Good or Bad for MAR:

MAR timing must balance three competing factors:

1. Natural Recharge Competition (Mar-May are BAD) - Spring = natural high recharge (snowmelt + rain) - Adding MAR during natural recharge = double-charging the aquifer - Risk: Water table rises too high → flooding, infrastructure damage - Rule: Avoid MAR when natural recharge >30 mm/month

2. Source Water Availability (Jun-Aug are GOOD) - Summer = low natural recharge BUT streams still have base flow - Stream environmental flow requirement: 40% of median (0.67 m³/s) - June excess: 1.43 m³/s (enough for all 10 sites) - July excess: 0.43 m³/s (enough for top 3 sites) - Rule: Operate MAR only when stream flow >environmental minimum

3. Physical Constraints (Jan-Feb, Dec are BAD) - Frozen ground (Dec-Feb) prevents infiltration - Equipment risk: Ice damage to pipes, valves - Worker safety: Unsafe conditions - Rule: No MAR operations when air temp <0°C for >7 days

Seasonal Capacity Limits:

Operating Window Available Water Sites You Can Run Max Capacity
June (30 days) 1.43 m³/s excess All 10 sites 459,000 m³
July (31 days) 0.43 m³/s excess Top 3 sites only 137,000 m³
August (31 days) 0.23 m³/s excess Rank 1 only 42,000 m³
May (31 days) 0.8 m³/s excess Top 7 sites 280,000 m³ (if approved)
September (30 days) 0.13 m³/s Rank 1 only 25,000 m³ (marginal)

Operational Calendar:

Primary Season (June-August, 90 days): - June (30d): Full system operation (all 10 sites) - July (31d): Partial operation (sites 1-3) - August (31d): Minimal operation (site 1 only) - Total recharged: ~640,000 m³

Extended Season (Add May, 120 days total): - May (31d): Conditional operation if natural recharge low - Requires monitoring: If Apr rainfall <30mm, approve May MAR - Additional: ~280,000 m³ - New total: ~920,000 m³

Why Not Operate Year-Round:

Many MAR systems operate year-round—why not here?

  • Spring (Mar-May): Aquifer already receiving 35-42 mm natural recharge—adding MAR risks over-saturation
  • Winter (Dec-Feb): Frozen ground, ice damage, safety issues
  • Fall (Oct-Nov): Fall rains (18-25 mm) provide natural recharge, less need for MAR

Adaptive Management Triggers:

Condition Action Reason
Drought year (rainfall <75% of normal) Extend to May + Sep (150 days) Natural recharge insufficient
Wet year (rainfall >125% of normal) Reduce to July-Aug only (60 days) Natural recharge adequate
Stream flow <0.67 m³/s (environmental min) Cease MAR immediately Protect aquatic habitat
Water table rise >3m Pause MAR until levels drop Prevent flooding

Key Insight: MAR is seasonal storage, not year-round supply. Operate 90-120 days/year, store water underground, pump it out during low-water periods (drought, winter).

48.6.1 Natural Recharge Pattern

Avoid competition with natural recharge: - High natural recharge: March-May (spring snowmelt) - Low natural recharge: June-August (high ET), December-February (frozen)

48.6.2 Optimal MAR Timing

Month Natural Recharge Stream Flow MAR Suitability Recommended
Jan Low (8 mm) Low (1.2 m³/s) 0.3 ❌ NO (frozen ground)
Feb Low (12 mm) Low (1.5 m³/s) 0.4 ❌ NO (limited water)
Mar High (35 mm) High (3.5 m³/s) 0.2 ❌ NO (compete with natural)
Apr High (42 mm) High (4.2 m³/s) 0.1 ❌ NO (compete with natural)
May High (38 mm) High (3.8 m³/s) 0.3 ⚠️ MARGINAL
Jun Low (15 mm) Moderate (2.1 m³/s) 0.9 ✅ EXCELLENT
Jul Low (8 mm) Low (1.1 m³/s) 0.8 ✅ GOOD
Aug Low (6 mm) Low (0.9 m³/s) 0.7 ✅ GOOD
Sep Low (10 mm) Low (0.8 m³/s) 0.6 ⚠️ MARGINAL (flow limited)
Oct Low (18 mm) Moderate (1.3 m³/s) 0.5 ⚠️ MARGINAL
Nov Moderate (25 mm) Moderate (2.0 m³/s) 0.4 ❌ NO (fall rains)
Dec Low (14 mm) Low (1.4 m³/s) 0.3 ❌ NO (freezing risk)

Recommended Operating Season: June-August (90 days) Secondary Season: May, September (60 days if needed) Total Operating Days: 90-150 days per year

48.7 Infrastructure Design

Tip📖 Understanding Design Parameter Choices

How Design Choices Were Made:

Each design parameter represents a trade-off between performance, cost, and risk.

Basin Size (1 hectare = 100m × 100m):

Size Option Infiltration Capacity Cost Trade-Off
0.5 ha (50×100m) Half capacity (425 m³/day) $180K (36% cheaper) Lower capacity, faster clogging
1 ha (100×100m) Full capacity (850 m³/day) $280K (selected) Balanced
2 ha (140×140m) Double capacity (1700 m³/day) $480K (71% more expensive) Over-capacity (exceeds source water)

Why 1 hectare: Matches source water availability (Jun-Aug average = ~700 m³/day). Larger basin wastes money; smaller basin limits capacity.

Basin Depth (2 meters total):

  • 1m operating depth: Water infiltrates from surface to 1m down
  • 1m freeboard: Safety margin prevents overflow during high-flow events
  • Why not deeper: Deeper basins cost more, don’t increase infiltration (rate determined by K, not depth)

Effective K with Clogging (6.25 m/day):

  • Raw K from HTEM: 12.5 m/day
  • Clogging factor: 0.5 (assumes 50% reduction over time from sediment, biofilms)
  • Why 50% reduction: Conservative based on literature (MAR basins degrade 30-70% over 5-10 years)
  • Mitigation: Periodic scraping (every 2-3 years) restores 80% of original K

Pipeline Diameter (450mm):

Diameter Flow Capacity Cost per Meter Total Cost (420m) Trade-Off
300mm 280 m³/day $140/m $59K Too small, limits capacity
450mm 900 m³/day $210/m $95K (selected) Adequate headroom
600mm 1500 m³/day $320/m $134K Over-capacity, waste $39K

Why 450mm: Provides 900 m³/day capacity (50% above current need) for future expansion without over-spending.

Gravity vs Pumped Flow:

  • Gravity flow (if stream elevation >2m above basin): $0/year operating cost, but requires specific topography
  • Pumped flow (if stream below basin): $15K/year electricity, but works anywhere
  • Design assumes: Check topography—use gravity if possible, include pump as contingency

Cost-Benefit of Design Choices:

Component Selected Design Cheaper Alternative Why Selected Design Better
Basin size 1 ha ($280K) 0.5 ha ($180K) $100K extra → 2× capacity → ROI 4.2 years vs 6.8 years
Pipeline 450mm ($95K) 300mm ($59K) $36K extra → Future-proof (no re-build if expand)
Monitoring wells 4 wells ($25K) 2 wells ($13K) $12K extra → 3D water level monitoring → Detect problems early

Scaling Considerations:

If you need to scale capacity up or down:

Scale DOWN (Budget <$200K): - Reduce basin to 0.7 ha → $220K total - Accept 600 m³/day capacity (still viable) - Delay monitoring wells to Year 2

Scale UP (Want max capacity): - Increase basin to 1.3 ha → $340K total - 1100 m³/day capacity - Justify only if source water exceeds 1.0 m³/s

Key Insight: Selected design optimizes for 90-day operating season at 850 m³/day, which matches source water availability. Over-designing wastes money; under-designing limits performance.

48.7.1 Site #1 (Top Ranked)

Configuration: Surface spreading basin

Dimensions: - Basin area: 1 hectare (100m × 100m) - Basin depth: 2 meters (1m operating depth + 1m freeboard) - Excavation volume: 20,000 m³

Hydraulic Design: - Effective K (with clogging): 12.5 m/day × 0.5 = 6.25 m/day - Infiltration rate: 6.25 m/day - Daily volume: 6.25 m/day × 10,000 m² = 62,500 m³/day - Adjusted for operation (12 hrs/day): ~850 m³/day

Conveyance: - Distance to stream: 420 meters - Pipeline: 450mm diameter, gravity flow (if elevation permits) - Pumping (if needed): 0.01 m³/s × 8 hrs/day = 290 m³/day capacity - Diversion structure: Automated gate with flow meter

Estimated Costs: - Excavation: $80,000 (at $4/m³) - Pipeline: $95,000 (at $210/m for 450mm) - Diversion structure: $45,000 - Inlet/outlet works: $35,000 - Monitoring wells (4): $25,000 - Total construction: $280,000

Annual Operating Costs: - Maintenance: $12,000/year - Water quality monitoring: $8,000/year - Electricity (if pumped): $15,000/year - Total operating: $35,000/year

48.8 Source Water Availability

Tip📖 What “Sufficient” Really Means

How to Interpret Water Availability:

“Sufficient source water” doesn’t mean unlimited—it means demand is within sustainable limits.

What the Numbers Mean:

Environmental Flow Requirement (0.67 m³/s = 40% of median): - Ecological threshold: Below this, stream habitat degrades (fish die, riparian stress) - Legal requirement: State environmental regulations prohibit withdrawals that drop flow below 40% median - Why 40%: Based on “Tennant Method” (ecological flow studies from 1970s-80s)

Available Water Calculation:

Available = Streamflow - Environmental Minimum
June: 2.1 - 0.67 = 1.43 m³/s available

Converting to Monthly Volume:

Monthly Volume = Available Flow × Seconds/Month
June: 1.43 m³/s × 30 days × 86,400 s/day = 3.7 million m³

Demand vs Supply:

Period Available Supply MAR Demand (Top 10) Margin % Utilized
June 3.7M m³ 0.5M m³ 3.2M m³ surplus 14%
July 1.2M m³ 0.5M m³ 0.7M m³ surplus 42%
August 0.6M m³ 0.5M m³ 0.1M m³ surplus 83%
Total (3 months) 5.5M m³ 1.5M m³ 4.0M m³ surplus 27%

Minimum Requirements for “Sufficient”:

  • ✅ Demand <50% of available (reduces competition, drought buffer)
  • ✅ No month exceeds 90% utilization (safety margin for variability)
  • ✅ Environmental flow never violated (legal compliance)

August is Critical Month:

  • Lowest flow (0.9 m³/s total)
  • Highest utilization (83% of available)
  • Risk: Dry August could drop streamflow to 0.7 m³/s → Only 0.03 m³/s available → Must reduce MAR

Safety Margins:

Condition Action Trigger
Normal operations All 10 sites operate Streamflow >1.0 m³/s
Low flow (drought) Reduce to sites 1-5 Streamflow 0.8-1.0 m³/s
Very low flow Site 1 only Streamflow 0.7-0.8 m³/s
Critical low flow Cease MAR Streamflow <0.7 m³/s

Drought Contingency:

In a 1-in-10 drought year: - August streamflow drops to 0.6 m³/s (historical minimum) - Available: 0.6 - 0.67 = NEGATIVE (below environmental min) - Action: Cease all MAR in August - Impact: Lose 16% of annual recharge (0.24M m³)

Long-Term Sustainability:

Over 20 years, expected performance: - Normal years (60%): Operate 90 days, recharge 1.5M m³ - Drought years (30%): Operate 60 days, recharge 1.0M m³ - Wet years (10%): Operate 120 days, recharge 2.0M m³ - 20-year average: 1.4M m³/year

Key Insight: “Sufficient” means we use only 27% of available water, leaving 73% for environmental flows and drought buffer. This ensures MAR is sustainable even in dry years.

48.8.1 Stream Discharge Analysis

Environmental Flow Requirement: 40% of median flow = 0.67 m³/s

Available for MAR (excess above environmental minimum):

Month Streamflow Environmental Available Monthly Volume
Jun 2.1 m³/s 0.67 m³/s 1.43 m³/s 3.7 million m³
Jul 1.1 m³/s 0.67 m³/s 0.43 m³/s 1.2 million m³
Aug 0.9 m³/s 0.67 m³/s 0.23 m³/s 0.6 million m³

Total Available (June-August): 5.5 million m³

Top 10 Sites Total Demand (June-August): 1.5 million m³

Conclusion: ✅ Sufficient source water to operate all top 10 sites simultaneously.

48.9 Impact Forecasting

Tip📖 Understanding MAR Impact Forecasts

What Forecasts Tell You:

Impact forecasting answers: “If we infiltrate X m³ of water, how much will nearby water levels rise, and how fast?”

Operational Meaning of Forecast Results:

Water Level Rise (+1.6m at 100m distance): - What it means: Well 100m from MAR basin will see water level rise from 15.2m to 16.8m depth - Is this good?: YES—raises water table, increases storage, makes pumping easier (shallower water) - Could it be bad?: If water level rises above 5m depth → Risk of flooding basements or surfacing - Monitoring trigger: If level rises >3m in any well → Reduce MAR rate

Time to Peak (45 days at 100m): - What it means: Maximum water level rise occurs 45 days after MAR starts - Why delay?: Water infiltrates slowly through unsaturated zone (vadose zone), then spreads laterally - Planning use: Don’t expect immediate benefits—allow 45-90 days for aquifer response

Distance Decay (+1.6m → +0.1m over 2000m): - What it means: MAR impact diminishes with distance (most benefit within 500m) - Why?: Aquifer transmissivity (T=285 m²/day) determines lateral spreading rate - Siting wells: Place production wells 500-1000m from MAR basin (get 0.3-0.5m boost)

Storage Added vs Infiltrated (95K / 153K = 62%): - What it means: Only 62% of infiltrated water stays in local aquifer storage - Where does 38% go?: - 20%: Lateral flow to regional aquifer (spreads beyond forecast area) - 10%: Evapotranspiration (plants uptake during growing season) - 8%: Deep percolation to lower units (loss to deeper confined aquifer) - Is 62% good?: YES—typical MAR recovery efficiency is 50-70%

Confidence Interpretation:

Forecast Element Confidence Level How to Use
Water level rise 97.2% accuracy (±0.15m) High confidence—use for planning
Time to peak ±7 days uncertainty Moderate confidence—allow buffer
Lateral extent ±200m uncertainty Lower confidence—verify with monitoring
Recovery efficiency ±10% uncertainty Plan for 52-72% range

Planning Horizons:

Forecast Period Use Case Reliability
1-30 days Operational decisions (when to turn on/off MAR) Very high (97%)
1-6 months Seasonal planning (how much to recharge this summer) High (90%)
1-5 years Long-term capacity planning (multi-year storage) Moderate (75%)
>5 years Climate change scenarios Low (50%)—too many unknowns

What to Monitor During Operations:

Measurement Frequency Purpose Alert Trigger
Water levels (4 wells) Daily Verify forecast accuracy Deviation >0.3m from predicted
Infiltration rate Hourly Detect clogging Drop >20% from baseline
Source water quality Weekly Prevent contamination Any exceedance of standards
Streamflow Real-time Protect environmental flows Drop below 0.7 m³/s

Key Insight: Forecasts provide expected outcomes, not guarantees. Use for planning, but verify with monitoring. If actual differs from forecast by >20%, investigate causes (clogging, unexpected aquifer heterogeneity, etc.).

48.9.1 Predictive Model

Question: How much will MAR raise water levels?

Method: Multi-modal fusion model (from Investigation 24) - Input: MAR infiltration rate, duration, location - Output: Water level response in nearby wells (1-30 day forecast) - Accuracy: 97.2%

Simulation (Site #1, 90-day operation):

Well Distance Baseline Level After MAR Change Time to Peak
100m (nearby) 15.2m 16.8m +1.6m 45 days
500m 14.8m 15.3m +0.5m 60 days
1000m 15.1m 15.4m +0.3m 75 days
2000m 14.9m 15.0m +0.1m 90 days

Storage Added: ~95,000 m³ (accounting for losses to lateral flow, ET)

Recovery Efficiency: 62% (95K / 153K infiltrated)

48.10 Cost-Benefit Analysis

NoteUnderstanding Cost-Benefit Analysis for MAR Projects

What Is It? Cost-benefit analysis (CBA) compares all project costs against all project benefits over the infrastructure lifetime (typically 20-50 years for water projects). This method was formalized by Jules Dupuit in the 1840s for bridge construction and became mandatory for U.S. federal water projects in the 1930s (Flood Control Act of 1936).

Why Does It Matter? MAR projects require large upfront investment ($200K-$500K per site) but deliver benefits over decades. CBA answers: “Is this investment worth it compared to alternatives (building reservoirs, buying water rights, doing nothing)?” It also helps prioritize among competing MAR sites when budgets are limited.

How Does It Work?

  1. List All Costs: Construction + annual operating costs for 20-30 years
  2. List All Benefits: Value of water stored (at avoided-cost or market price)
  3. Convert to Present Value: Apply 5% discount rate to future $ amounts
  4. Calculate NPV: Benefits - Costs (all in present-value terms)
  5. Calculate B/C Ratio: Benefits ÷ Costs (>1.0 means profitable)
  6. Calculate Payback Period: Years until cumulative benefits exceed costs

How to Interpret B/C Ratio:

B/C Ratio Meaning Investment Decision Typical Project
>5.0 Highly favorable Strong YES—prioritize funding Top MAR site: 5.1:1
3.0-5.0 Very favorable YES—good investment Well-designed infrastructure
1.5-3.0 Favorable YES if funds available Marginal sites or expensive regions
1.0-1.5 Marginal Maybe—depends on non-financial factors Social equity projects
<1.0 Unfavorable NO—costs exceed benefits Poor site selection

Payback Period Interpretation:

Payback Period Meaning Risk Assessment
<5 years Very fast payback Low risk—recover investment quickly
5-10 years Moderate payback Acceptable for water infrastructure
10-20 years Slow payback Typical for large projects, consider carefully
>20 years Very slow payback High risk—vulnerable to changing conditions

Key Assumptions to Question: - Avoided cost ($2.80/m³): Based on alternative water supply cost (surface reservoir or water rights purchase) - Recovery efficiency (62%): Not all infiltrated water is recoverable (some flows away laterally) - Operating life (20 years): Assumes basin doesn’t clog or become unusable - Discount rate (5%): Lower rate favors long-term projects, higher rate favors short-term

Sensitivity Analysis: Test how NPV changes if assumptions vary: - Avoided cost ±20%: NPV ranges $2.2M - $3.7M (still positive) - Recovery efficiency ±10%: NPV ranges $2.5M - $3.4M (still favorable) - Clogging reduces capacity 30%: NPV drops to $2.1M (still acceptable)

System-Wide vs Individual Site: System-wide B/C ratio (10.7:1) is higher than individual sites (5.1:1) due to shared infrastructure (pipelines, diversion structures) and economies of scale.

48.10.1 20-Year NPV (Site #1)

Costs: - Initial construction: $280,000 - Annual operating: $35,000/year - Present value (20 yr, 5% discount): $280K + $436K = $716K

Benefits: - Water stored: 95,000 m³/year × 20 years = 1.9 million m³ - Avoided cost (vs surface reservoir): $2.80/m³ - Present value of benefits: 1.9M × $2.80 × discount = $3.68M

Net Present Value: $3.68M - $0.72M = $2.96 million

Benefit-Cost Ratio: 5.1:1

Payback Period: 4.2 years

48.10.2 System-Wide Economics

Total Construction: $68.5 million

Total Annual Recharge: 21.4 million m³

NPV (20 years): $731 million

B/C Ratio: 10.7:1 (economies of scale)

48.11 Risk Assessment

NoteUnderstanding Risk Assessment for MAR Projects

What Is It? Risk assessment is a systematic framework for identifying, quantifying, and mitigating potential project failures. This approach emerged from nuclear engineering safety analysis (1960s-70s) and was adapted for environmental projects in the 1980s-90s. For MAR, we assess technical, hydrological, financial, and regulatory risks.

Why Does It Matter? MAR projects can fail in multiple ways: infiltration basins clog faster than expected, source water becomes unavailable during droughts, contamination shuts down operations. Risk assessment identifies these failure modes BEFORE construction, allowing mitigation strategies. It also helps set realistic expectations with stakeholders and justifies contingency budgets.

How Does It Work?

  1. Identify Risks: List all potential failure modes (clogging, drought, contamination)
  2. Assess Probability: Estimate likelihood based on historical data (10-70%)
  3. Assess Impact: Rate severity if risk occurs (Low/Moderate/High)
  4. Calculate Risk Level: Probability × Impact = Risk score
  5. Prioritize Mitigation: Focus on high-probability AND high-impact risks first
  6. Monitor: Track risk indicators during operations (infiltration rate, source water quality)

How to Read the Risk Matrix:

Probability × Impact Risk Level Management Approach Example
High × High CRITICAL Must mitigate before proceeding Contamination (low prob but catastrophic)
High × Moderate HIGH Active mitigation required Clogging (70% prob, moderate impact)
Moderate × High HIGH Prepare contingency plans Source water shortage (30% prob)
Moderate × Moderate MEDIUM Monitor and respond Cost overruns (35% prob)
Low × High MEDIUM Insurance or reserves Contamination (10% prob)
Moderate × Low LOW Accept risk Permitting delays (annoyance, not critical)

Probability Definitions: - High (>50%): More likely to occur than not—plan as if it will happen - Moderate (20-50%): Significant chance—prepare contingency - Low (<20%): Unlikely but possible—monitor indicators

Impact Definitions: - High: Project failure or >50% capacity loss—threatens viability - Moderate: 20-50% capacity loss—reduces benefits but project continues - Low: <20% impact—nuisance but manageable

Mitigation Strategies Effectiveness:

Risk If Unmitigated With Mitigation Residual Risk
Clogging 70% prob, 50% capacity loss Pre-treatment + scraping → 30% prob, 20% loss ACCEPTABLE
Source shortage 30% prob, 100% loss for season Prioritize high-B/C sites → Same prob, 50% loss MONITOR
Contamination 10% prob, 100% shutdown Continuous monitoring + shutoff → 2% prob, 0% loss ACCEPTABLE
Cost overruns 35% prob, 15% over budget 20% contingency fund → Budget protected ACCEPTABLE

Key Insight: High-probability risks (clogging) require active mitigation, while high-impact risks (contamination) require monitoring and emergency response plans. The combination determines management approach.

48.11.1 Uncertainties

Risk Factor Probability Impact Mitigation
Clogging (50% K reduction) High (70%) Moderate Pre-treatment, periodic scraping
Source water shortage (drought) Moderate (30%) High Prioritize high-return sites
Contamination (poor source water) Low (10%) High Continuous monitoring, shutoff valves
Permitting delays Moderate (40%) Low Early engagement, phased approach
Cost overruns (15% over budget) Moderate (35%) Moderate Contingency fund (20%)

48.11.2 Adaptive Management

Monitor quarterly: - Infiltration rates (declining = clogging) - Water quality (source and aquifer) - Water level response (verify model predictions) - Cost tracking (vs budget)

Adjust operations: - If clogging: Reduce flow rate, schedule maintenance - If contamination: Switch to alternate source or cease - If unexpected impacts: Modify infiltration schedule

48.12 Implementation Roadmap

48.12.1 Phase 1 Pilot

  • Construct top 3 sites (total capacity: 459,000 m³/year)
  • Cost: $840K
  • Validate models, refine designs
  • Train operators, establish protocols

48.12.2 Phase 2 Expansion

  • Construct next 7 sites (additional 1.04M m³/year)
  • Cost: $1.96M
  • Economies of scale (shared infrastructure)
  • Regional coverage

48.12.3 Phase 3 Full Deployment

  • Construct remaining high-priority sites (as budget allows)
  • Integrate with groundwater management plan
  • Continuous optimization based on performance

48.13 Production Deployment Checklist

Status: ⚠️ Field validation required before construction. Models ready for design.

Framework Version: MAR Siting v1.0 Analysis Date: 2024-11-26 Candidate Sites: 247 (25% of study area) Top Site Capacity: 153,000 m³/year System Capacity: 21.4 million m³/year Next Step: Field verification (soil borings at top 10 sites) Responsible: Planning + Hydrogeology + Engineering

48.14 Reflection Questions

  1. Given the eight suitability factors and their weights, which factors would you consider non-negotiable in your own basin, and which could you relax if budgets or data are limited?
  2. How might changing the distance-to-source or land-availability thresholds alter the spatial pattern of “Excellent” vs “Good” MAR sites?
  3. Looking at the seasonal operating strategy and source water analysis, what additional hydrologic or ecological constraints would you want to include before finalizing operations?
  4. Where do you think model uncertainty (e.g., K, VCI, recharge estimates) could most strongly affect MAR outcomes, and how would you design field investigations to reduce that uncertainty?
  5. How would you explain the benefits, risks, and trade-offs of a MAR program—based on these analyses—to community stakeholders and regulators?