Aquifer Vulnerability Assessment
Methods for evaluating how susceptible groundwater is to contamination from surface activities.
Why Vulnerability Assessment?
Not all groundwater is equally vulnerable to contamination. Understanding vulnerability helps prioritize protection efforts, guide land-use planning, and allocate monitoring resources.
DRASTIC Method
The most widely used vulnerability assessment method — developed by US EPA (Aller et al., 1987):
Seven Parameters
| Parameter | Description | Rating Scale | Weight |
|---|---|---|---|
| Depth to water | Depth from surface to water table | 1-10 (deeper = lower) | 5 |
| Recharge | Amount of water reaching water table | 1-10 (more = higher) | 4 |
| Aquifer media | Geological material of aquifer | 1-10 (coarse = higher) | 3 |
| Soil media | Upper 1-2 m of soil | 1-10 (sandy = higher) | 5 |
| Topography | Slope steepness | 1-10 (steep = higher) | 1 |
| Impact of vadose zone | Material between soil and water table | 1-10 (permeable = higher) | 3 |
| Conductivity (K) | Hydraulic conductivity of aquifer | 1-10 (high K = higher) | 3 |
DRASTIC Index Calculation
DRASTIC Index = Σ (Rating × Weight)
DI = D×5 + R×4 + A×3 + S×5 + T×1 + I×3 + C×3
Vulnerability Classes:
DI < 100: Low vulnerability (green)
DI 100-150: Moderate vulnerability (yellow)
DI 150-180: High vulnerability (orange)
DI > 180: Very high vulnerability (red)
DI = D×5 + R×4 + A×3 + S×5 + T×1 + I×3 + C×3
Vulnerability Classes:
DI < 100: Low vulnerability (green)
DI 100-150: Moderate vulnerability (yellow)
DI 150-180: High vulnerability (orange)
DI > 180: Very high vulnerability (red)
Rating Tables (Selected Values)
| Depth to Water (m) | Rating | Aquifer Media | Rating |
|---|---|---|---|
| 0-1.5 | 10 | Gravel | 10 |
| 1.5-3 | 9 | Coarse sand | 8 |
| 3-6 | 7 | Fine sand | 6 |
| 6-9 | 5 | Sandstone | 4 |
| 9-15 | 3 | Limestone | 3 |
| >15 | 1 | Shale/clay | 1 |
GOD Method
Simpler alternative to DRASTIC — uses only three parameters:
- G — Groundwater occurrence: Confined (1), semi-confined (3), unconfined (5-9)
- O — Overlying layers: Soil and vadose zone material (1-10)
- D — Depth to water: Depth below surface (1-10)
GOD Index = G × O × D
GOD < 3: Low vulnerability
GOD 3-10: Moderate vulnerability
GOD 10-20: High vulnerability
GOD > 20: Very high vulnerability
GOD < 3: Low vulnerability
GOD 3-10: Moderate vulnerability
GOD 10-20: High vulnerability
GOD > 20: Very high vulnerability
Aquifer Vulnerability Index (AVI)
- Method: Uses thickness and hydraulic conductivity of layers above aquifer
- Formula: AVI = Σ (d_i / K_i) — sum of thickness/conductivity for each layer above aquifer
- Interpretation: Lower AVI = more vulnerable (less resistance to contamination)
- Data required: Borehole logs with layer thicknesses and K values
Source Water Protection Planning
- Wellhead protection areas: Zones around public water supply wells where activities are restricted
- Time-of-travel zones: 1-year, 5-year, 10-year, 25-year capture zones — nested rings
- Zone restrictions: Prohibit high-risk activities (fuel storage, chemical handling, waste disposal)
- Contingency planning: Response plans if contamination reaches protected zone
- Monitoring: Network of monitoring wells in protection zone — early warning
| Zone | Time-of-Travel | Restrictions |
|---|---|---|
| Zone 1 (Immediate) | 0-1 year | Strictest — no injection, storage, or discharge |
| Zone 2 | 1-5 years | Prohibited activities — fuel, chemicals, waste |
| Zone 3 | 5-10 years | Conditional activities — with spill prevention |
| Zone 4 (Buffer) | 10-25 years | Best management practices — monitoring required |
Land-Use Planning Applications
- Zoning overlays: Overlay high-vulnerability zones on land-use maps — guide development
- Environmental review: Require vulnerability assessment for new developments in high-risk areas
- Agricultural BMPs: Buffer strips, nutrient management plans in vulnerable areas
- Industrial siting: Locate high-risk facilities on low-vulnerability zones
- Septic system restrictions: Prohibit or require enhanced treatment in high-vulnerability zones
Vulnerability vs Risk
Vulnerability = intrinsic susceptibility of groundwater (DRASTIC). Risk = vulnerability × hazard (contamination source). A highly vulnerable area with no contamination sources has low risk. A low-vulnerability area with intensive industry may have high risk. Both must be assessed for effective protection.