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The DRASTIC model was developed by the US EPA as a way of quantifying an aquifer’s susceptibility to surface contamination events. In 1997, the then NDDoH (now NDDEQ) applied this system to North Dakota’s shallow aquifers as part of the North Dakota Geographic Targeting System for Groundwater Monitoring (GTS). In 2015, it was decided to update the GTS to reflect changes in aquifer boundaries, water use, and land use on a regular basis. The most recent report is available online. (URL https://deq.nd.gov/publications/wq/1_GW/GeoTargetSys/2018_GTS/2019_GTS_Report_October2019.pdf). The DRASTIC model was reapplied to all known shallow groundwater resources within North Dakota in order to obtain the vulnerability rating shown in Attachment 3 of the report.
The DRASTIC model consists of seven weighted factors:
1. Depth to Water (feet below ground surface)
2. Net Recharge (inches per year)
3. Aquifer Media (sand and gravel, clay, silt, etc.)
4. Soil Media (Loam, Silty Loam, Sand, etc.)
5. Topography (Percent Slope)
6. Impact of the Vadose Zone (Silt/Clay, Confining Layer, etc.)
7. Hydraulic Conductivity (cubic feet per year)
Each factor has two associated fields. These fields represent the actual unit value of the factor, and the second field represents the score* assigned to the factor. A score is a rating based on a scale of values, used along with a weighting factor to generate a final DRASTIC score.
The final two fields, DRASTIC and P_DRASTIC represent two methods of rating aquifers. The DRASTIC value is a general assessment of contamination susceptibility, while the P_DRASTIC represents a score adjusted to better represent susceptibility to pesticide use contamination. This is generally referred to as a “Rating” in the DRASTIC methodology but has been changed here to remain consistent with the GTS methods of differentiating between values, scores, and ratings. See North Dakota Geographic Targeting System for Groundwater Monitoring for additional information.
In December of 2018, an additional assessment was conducted on the water levels of confined aquifers to account for hydrostatic pressure, primarily addressing the Spiritwood Aquifer System. These changes are reflected in the December, 2018 updated GTS report.
In January 2021, 68 new aquifers were added to the layer that were included in the "2021 Addendum to the 2019 North Dakota Geographic Targeting System For Groundwater Monitoring". Four aquifer names were also updated and all aquifer boundaries were updated to match North Dakota State Water Commission aquifer delineations and names as of January 2021. The 2021 addendum report can be found online at https://deq.nd.gov/WQ/publications.aspx under "Groundwater Protection Program."