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Determination of Aquifer Vulnerability using DRASTIC model across conterminous United States By Karthik Kumarasamy Final Presentation of GIS in Water Resources Utah State University Overview • Concept of groundwater vulnerability • DRASTIC Model • Data Sources and Preparation • Comparison with Nitrate concentration • Conclusion Concept of groundwater vulnerability Purpose: Management Tool to protect groundwater resources Definition: Probability that a specific contaminant will be detected at or above a specified concentration in the subsurface at a specific location (EPA definition) Some Vulnerability assessment models – – – – – DRASTIC AVI GOD SEEPAGE Ordinal Logistic Regression, etc DRASTIC Model DRASTIC Empirical model developed by National Water Well Association and EPA Parameters considered in DRASTIC – Depth to water table – Net Recharge – Aquifer media – Soil media – Topography (slope) – Impact of vadose zone media – (Aquifer hydraulic) Conductivity Final DRASTIC Index = 5D + 4R + 3A + 2S + 1T + 5I + 3C Study Area: Conterminous United States Depth to water table Source: STATSGO Database Methodology: • Link the attribute table of Each State and component table to get the (Maximum value for the range in depth to seasonally high water table) • Append each state to get the final US map • (These steps are common for all maps using STATSGO data) • The field is known as wtdeph Ratings of depth to water table Net Recharge • Net recharge or groundwater recharge is estimated using Williams and Kissel's equation: – – – – – – – PI = (P - 10.28)2/(P + 15.43) for hydrologic soil group A PI = (P - 15.05)2/(P + 22.57) for hydrologic soil group B PI = (P - 19.53)2/(P + 29.29) for hydrologic soil group C PI = (P - 22.67)2/(P + 34.00) for hydrologic soil group D PI = average of A and D for Hydrologic soil group A/D PI = average of B and D for Hydrologic soil group B/D PI = average of C and D for Hydrologic soil group C/D where, » PI = Percolation index » P = Annual average precipitation Ratings of Net Recharge DATA Source: – PRISM for precipitation data – STATSGO for soil hydrologic group Ratings of Net Recharge Ratings for Aquifer Media Source: Shallow aquifers of conterminous United States (Groundwater atlas of US) •Other Rocks: 3 •Carbonate-rock aquifers: 8 •Igneous and metamorphic rock aquifers: 3 •Sand stone and carbonate rock aquifers: 6 •Sandstone aquifers: 6 •Semiconsolidated sand aquifers: 4 •Unconsolidated sand and gravel aquifers: 8 Rating of Aquifer media Rating for Soil Media (Done based on the Soil Hydrologic group) • Source: STATSGO database • • • • • • • A: 8 B: 5 C: 4 D: 3 A/D: 6 B/D: 4 C/D: 4 Rating of Soil Media Topography Topography (slope): STATSGO soil database Methodology (sloepl + slopeh) /2 * (comppct/100) Ratings Rating of Topography Impact of vadose zone • Data from Groundwater Atlas (shallowest principal aquifers of the conterminous United States) to determine the Vadose Zone ratings • Assuming that the same geology will be present above the water table as well Rating for Impact of Vadose zone • Other Rocks: 1 • Carbonate-rock aquifers: 6 • Igneous and metamorphic rock aquifers: 4 • Sand stone and carbonate rock aquifers: 6 • Sandstone aquifers: 6 • Semiconsolidated sand aquifers: 5 • Unconsolidated sand and gravel aquifers: 8 Rating for Impact of Vadose zone Ranges for Hydraulic Conductivity (GPD/ ft2) (Freeze and Cheery, 1979) • • • • • • • Other Rocks: little or no permeability Carbonate-rock aquifers: 10-2 – 101 Igneous and metamorphic rock aquifers: 10-7 – 103 Sand stone and carbonate rock aquifers: 10-3 – 101 Sandstone aquifers: 10-3 – 101 Semiconsolidated sand aquifers: 1-103 Unconsolidated sand and gravel aquifers: 102 - 106 Ratings for Hydraulic Conductivity • • • • • • • Other Rocks: Carbonate-rock aquifers: 1 Igneous and metamorphic rock aquifers: 1 Sand stone and carbonate rock aquifers: 1 Sandstone aquifers: 1 Semiconsolidated sand aquifers: 8 Unconsolidated sand and gravel aquifers: 10 Ratings of Hydraulic Conductivity DRASTIC Index Nitrate concentration Data • Source: usgs (NAWQA program) • Values are the median values of NO3 concentration Distribution of Nitrate concentration across United States Conclusion • Areas shown vulnerable by the DRASTIC model spatially coincide with nitrate concentration wells with high concentration of Nitrate • Even though there is subjectivity in the rating of parameters results match well Thank you!!!! Any Questions