1. DRASTIC Source Water Assessment Method for
High Plains Aquifer Units on the Pine Ridge Reservation
in Southwestern South Dakota
Lilly M. Jones, South Dakota School of Mines and Technology, Dr. J. Foster Sawyer, Advisor, South Dakota School of Mines and Technology
Abstract
Aquifer vulnerability assessment is used by scientists, and water-resource managers to guide management of groundwater resources. Assessment parameters include proximity of contaminants to the groundwater system, contaminant load, factors which could increase contaminant load, geochemical properties of contaminants, and fate/transport of contaminants in a groundwater system. Subjective methods use ratings systems, while objective assessments combine statistical or process-based methods such as modeling, regression equations, and map overlay to determine relationships between explanatory variables or processes.
Most source water wells on the Pine Ridge Reservation are completed in the Arikaree aquifer. Drinking water is also pumped from the Ogallala aquifer; this water is then mixed 50/50 with water piped from the Missouri River. The Arikaree and Ogallala units comprise the northernmost extent of the High Plains Aquifer. Hydrogeologic models have been produced for the study area by the U.S. Geological Survey but as of this date, a comprehensive source-water assessment for the Pine Ridge Reservation has not been completed.
The goal of this vulnerability assessment is to produce scientifically defensible products that can be shared with stakeholders. A process-based model published by the US Geological Survey in 2014 will provide the basis for objective assessment. Site-specific contaminant and water quality data will be added to the model to complete the assessment. Combining a process-based model with measured water quality data and contaminant-specific parameters in a map-overlay process will produce source water assessment products with objective measures of aquifer vulnerability for the Pine Ridge Reservation.
Figure 2
Figure 3
Methods
Methods to assess groundwater vulnerability to contamination can be subjective or objective. Subjective methods define categories of vulnerability (low, medium, high) based on a rating system. Index-overlay rating methods such as DRASTIC (Aller and others, 1985) were developed to provide an inexpensive means of source water assessment that uses known hydrogeologic parameters in cases where time or budget constraints inhibit extensive, site-specific data collection. Resources can then be allocated for further investigation at sites identified as sensitive by the DRASTIC assessment.
Parameters used for the DRASTIC method include: depth to groundwater (D), net recharge (R), aquifer media (A), soil media (S), topography (T), impacts of the vadose zone (I), and hydraulic conductivity (C). For this project, each of these layers was built in ArcGIS Data points from each layer were assigned ratings according to the DRASTIC method (Aller and others, 1985) and rasters produced in order to perform raster calculations according to the DRASTIC index:
(Dr * Dw) + (Rr * Rw) + (Ar * Aw) + (Sr * Sw) + (Tr * Tw) + (Ir * Iw) + (Cr * Cw)
where r = the rating for the parameter and w = is an assigned weight for the parameter.
Drawbacks to using the DRASTIC method include assumptions about potential contaminants to an aquifer such as class of contaminant and method of infiltration, as well as assumptions about the aquifer and study area. DRASTIC is generally limited to unconfined aquifers in areas less than 100 acres. This method is also limited in that it relies on a subjective rating system subject to human error during both initial data collection, and while assigning the ratings.
Due to the size of the study area (4,355 mi2) and the incompleteness of the dataset, the DRASTIC method is used here to make a general identification of sensitive areas so that site-specific field work can be directed to those areas. The DRASTIC method was chosen as it is inexpensive, easy, uses available data, and produces an end product that water-resource decision makers can interpret and use.
Figure 1. Parameters used for the DRASTIC method include: depth to groundwater (D), net recharge (R), aquifer media (A), soil media (S), topography (T), impacts of the vadose zone (I), and hydraulic conductivity (C). For this project, each of these layers was built in ArcGIS
Conclusions
Citations
The goal of the DRASTIC assessment is to produce a preliminary sensitivity assessment that can be used to identify sites that require further field study, such as well testing.
As an assessment method, DRASTIC is too limited and subjective to provide scientifically defensible source water vulnerability products.
A process-based model was published by the US Geological Survey in 2014; this can provide the basis for assessing intrinsic vulnerability of High Plains aquifer units on the Pine Ridge Reservation based on hydraulic properties.
Combining this process-based model with measured well data and contaminant-specific parameters in a map-overlay process will produce aquifer vulnerability products with objective measures of aquifer vulnerability to contamination.
Aller, L., Bennett, T., Lehr, J.H., and Petty, R.J., 1985, DRASTIC- A standardized system for evaluating ground water pollution potential using hydrogeologic settings. U.S. Environmental Protection Agency report EPA/600/2-85/018, 163 p.
Carter, J.M., and Heakin, A.J., 2007, Generalized potentiometric surface of the Arikaree aquifer, Pine Ridge Indian Reservation and Bennett County, South Dakota: U.S. Geological Survey Scientific Investigations Map 2993, 2 sheets.
Davis, K.W., Putnam, L.D., and LaBelle, A.R., 2014, Conceptual and Numerical Models of Groundwater Flow in the Ogallala and Arikaree Aquifers, Pine Ridge Indian Reservation Area, South Dakota, Water Years 1980–2009, U.S. Geological Survey Scientific Investigations Report 2014–5241, 82 pp.
Heakin, A.J., 2000, Water quality of selected springs and public-supply wells, Pine Ridge Indian Reservation, South Dakota, : U.S. Geological Survey Water-Resources Investigations Report , 61 p.
Helsel, D.R. and Hirsch, R.M., 1992, Statistical Methods in Water Resources. Elsevier Publishers, New York, 529 p.
Martin, J. E., Sawyer, J. F., Fahrenbach, M. D., Tomhave, D. W., and Schulz, L. D., 2004, Geologic Map of South Dakota: South Dakota Geological Survey, GM 10. Scale 1:500,000.
National Research Council, 1993, Ground water vulnerability assessment, contamination potential under conditions of uncertainty: National Academy Press, Washington, D.C., 210 p. Accessed September 21, 2015 at:
U.S. Environmental Protection Agency, 1993a, A review of methods for assessing aquifer sensitivity and ground water vulnerability to pesticide contamination. U.S. Environmental Protection Agency, EPA/813/R-93/002, 147 p.
, State source water assessment and protection programs guidance. Final Guidance U.S. Environmental Protection Agency, EPA816-R , 127 p.

2. Assessing Arikaree Aquifer Vulnerability on the Pine Ridge Indian Reservation in Southwestern South Dakota
Lilly M. Jones, South Dakota School of Mines and Technology, Dr. J. Foster Sawyer, Advisor, South Dakota School of Mines and Technology
Abstract
Groundwater is used as a domestic water supply by about one-half of the US population (Alley and others, 1999). The U.S. Environmental Protection Agency (USEPA) requires assessment of groundwater vulnerability to contamination for all public water-supply systems as part of the Safe Drinking Water Act (1996).
Groundwater vulnerability assessment is a tool used by scientists, and federal, state, and local water-resource managers to guide management of water resources. Science objectives with regard to a vulnerability assessment involve reproducible, objective approaches that provide scientifically defensible data to water resource managers, regulators, and decision makers.
Assessment parameters include proximity of point source and non-point source contaminants to the groundwater system, contaminant load, factors which could increase contaminant load, geochemical properties of contaminants, and fate/transport of contaminants in a groundwater system. Objective assessments combine statistical or process-based methods such as modeling, regression equations, and map overlay to determine relationships between explanatory variables or processes that could cause aquifer contamination.
Most source water wells on the Pine Ridge Indian Reservation are completed in the Arikaree aquifer. Aquifer characteristics have been investigated by the U.S. Geological Survey but as of this date, a comprehensive source-water assessment for the Pine Ridge Reservation has not been completed.
The goal of this proposed vulnerability assessment is to produce scientifically defensible results that can be shared with stakeholders. A process-based model published by the US Geological Survey in 2014 will provide the basis for assessment. Contaminant and water quality data will be added to the model to complete the assessment. Combining a process-based model with measured water quality data and contaminant-specific parameters in a map-overlay process will produce source water assessment products with objective measures of aquifer vulnerability for the Pine Ridge Reservation.
Figure 2
Figure 3
Methods
Figure 1
Methods to assess groundwater vulnerability to contamination can be subjective or objective. Subjective ratings methods such as DRASTIC (Aller and others, 1985) are most commonly used by water-resource decision makers. Subjective methods define categories of vulnerability (low, medium, high) based on a rating system. These methods are inexpensive, easy, use available data, and produce an end product that water-resource decision makers can interpret and use.
Objective methods combine statistical or process-based methods to determine relationships between explanatory variables or processes that could lead to aquifer contamination. Statistical methods range from summary or descriptive statistics to regression analyses that incorporate relationships between predictor variables.
Regression equations can predict probabilities of contamination, producing weighting factors that are based on statistics rather than subjective ratings. Logistic regression involves statistically associating maps of measured data with maps of potential explanatory variables (Helsel and Hirsch, 1992), returning a hypothesis test for each independent variable. Insignificant values are dropped; the remaining values can be objectively weighted by their slope coefficients to estimate probability that values are statistically significant and reveal relationships between variables.
Process-based methods such as MODFLOW (Harbaugh and others, 2000) simulate physical processes (movement of water and contaminants) and may be calibrated with empirical data. MODFLOW uses the governing equations of groundwater flow (such as Darcy’s Law) to solve the three-dimensional, time dependent partial-differential equations that determine groundwater flow. Results can be used to model interacting components of a groundwater system and to predict aquifer vulnerability to contamination.
Conclusions
Citations
Aller, L., Bennett, T., Lehr, J.H., and Petty, R.J., 1985, DRASTIC- A standardized system for evaluating ground water pollution potential using hydrogeologic settings. U.S. Environmental Protection Agency report EPA/600/2-85/018, 163 p.
Carter, J.M., and Heakin, A.J., 2007, Generalized potentiometric surface of the Arikaree aquifer, Pine Ridge Indian Reservation and Bennett County, South Dakota: U.S. Geological Survey Scientific Investigations Map 2993, 2 sheets.
Davis, K.W., Putnam, L.D., and LaBelle, A.R., 2014, Conceptual and Numerical Models of Groundwater Flow in the Ogallala and Arikaree Aquifers, Pine Ridge Indian Reservation Area, South Dakota, Water Years 1980–2009, U.S. Geological Survey Scientific Investigations Report 2014–5241, 82 pp.
Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000, MODFLOW-2000, The U.S. Geological Survey Modular Ground-Water Model – Users guide to modularization concepts and the ground-water flow process. U.S. Geological Survey Open-File Report , 121 p.
Heakin, A.J., 2000, Water quality of selected springs and public-supply wells, Pine Ridge Indian Reservation, South Dakota, : U.S. Geological Survey Water-Resources Investigations Report , 61 p.
Helsel, D.R. and Hirsch, R.M., 1992, Statistical Methods in Water Resources. Elsevier Publishers, New York, 529 p.
Hoganson, J.W., Murphy, E.C., and Forsman, N.F., 1998, Lithostratigraphy, paleontology, and biochronology of the Chadron, Brule, and Arikaree Formations in North Dakota, in Terry, D.O., LaGarry, H.E., and Hunt, R.M., Jr., eds., Depositional Environments, Lithostratigraphy, and Biostratigraphy of the White River and Arikaree Groups: Geological Society of America Special Paper 325, p. 185–196
Martin, J. E., Sawyer, J. F., Fahrenbach, M. D., Tomhave, D. W., and Schulz, L. D., 2004, Geologic Map of South Dakota: South Dakota Geological Survey, GM 10. Scale 1:500,000.
National Research Council, 1993, Ground water vulnerability assessment, contamination potential under conditions of uncertainty: National Academy Press, Washington, D.C., 210 p. Accessed September 21, 2015 at:
U.S. Environmental Protection Agency, 1993a, A review of methods for assessing aquifer sensitivity and ground water vulnerability to pesticide contamination. U.S. Environmental Protection Agency, EPA/813/R-93/002, 147 p.
, State source water assessment and protection programs guidance. Final Guidance U.S. Environmental Protection Agency, EPA816-R , 127 p.
The goal of this vulnerability assessment is to produce scientifically defensible results that can be shared with tribal water-resource managers.
A process-based model was published by the US Geological Survey in 2014; this can provide the basis for assessing intrinsic vulnerability of the Arikaree aquifer on the Pine Ridge Reservation.
Combining a process-based model with measured well data and contaminant- specific parameters in a map-overlay process will produce aquifer vulnerability products with objective measures of aquifer vulnerability to contamination.
This poster shows an initial product that classifies Arikaree wells on Pine Ridge Reservation by hydraulic head (depth to top of well) to demonstrate one measure of intrinsic vulnerability. Directional distribution shows the directional mean of wells in the study area. This can be used to determine which wells to sample for water quality parameters. Additional products will be produced as this project evolves.