How vulnerable is each watershed to groundwater contamination?
Why is this important for geomorphology?
Groundwater Contamination Susceptiblity Health Scores
The inherent vulnerability of groundwater to contamination is based upon an understanding of how the shape, type, and relative position of surface and subsurface geology (e.g., the geomorphic setting) influences water movement. The geomorphic setting controls the flowpaths of water and dissolved elements above and below the surface. Sandiness, permeability, depth to rock outcrops, fracturing and permeability of bedrock, flow restricting layers, and surface and subsurface connections all affect how vulnerable groundwater is to contamination from surface spills, leaking storage tanks, or other sources of waterborne pollution.
CREATING THE INDEX
This index is based on the Groundwater Contamination Susceptibility model by the Minnesota Pollution Control Agency (Porcher, 1989). The vulnerability of groundwater to contamination was modeled based on the following inputs: aquifer materials, vadose zone (subsurface unsaturated layer) materials, net recharge, and soil type. The original susceptibility model was made from digital files with a useable map scale of approximately 1:500,000, or 1 inch = 8 miles, and was generated as a regional-scale screening aid for susceptibility. The original model ranked the susceptibility from very low to very high on scale of 0-4.
For this index, an area weighted mean value for each watershed was calculated based on the original 0-4 scale from the Groundwater Contamination Susceptibility model, rescaled from 100 (least susceptible) to 0 (most susceptible). The watershed score in this index represents an average susceptibility score per watershed.
The original numerical values associated with each susceptibility ranking were used to create an area weighted mean value for each watershed. Because there is insuffient data in the scientific literature to numerically rank these data and there are no known threshold values, the resulting values were ranked in equal intervals from 0 (least desirable condition) to 100 (most desirable condition).
The values were scored as followed:
- 0-20 = Very high susceptibility
- 21-40 = High susceptibility
- 41-60 = Moderate susceptibility
- 61-80 = Slight susceptibility
- 81-100 = Very slight susceptibility
The lowest values (high susceptibility) were in the karst region of the southeast along a diagonal toward the northwest. Most watersheds have moderate susceptibilitywith the highest scores (slight susceptibility) in the watersheds along the north and northeast.
The original model of ground water susceptibility (image above) shows a varying pattern of vulnerability across the state. Areas of highest ground water contamination susceptibility are in central, north-central, and east-central Minnesota in addition to the southeastern corner of the State in areas dominated by sand and gravel aquifers or in areas with karstic bedrock (Porcher, 1989). The highest susceptibility areas have high proportions of sand, gravel, sandstone, and/or karstic limestone, which are generally associated with moderate to high potential rates of recharge from surface or subsurface water sources. Since soil and subsurface deposits are generally course-textured and subsurface rocks do not form basin-sealing layers, water moves more easily both vertically downward from the surface and horizontally through the deposits. Furthermore, porous rock layers are generally found in these regions: either relatively porous or fractured sandstone, or karstic limestone with characteristic subsurface to surface channels.
The areas with the lowest ground water contamination susceptibility have opposite characteristics including: relatively impermeable surface deposits that reduce infiltration of surface water, low yielding aquifer materials with higher portions of clays and finer textured deposits at depth, and a very low to low recharge potential with reduced rates of water movement into or out of the deposits.
At a watershed scale, a high risk for groundwater contamination is prevalent in the karst landscape of the southeast. Other patterns of susceptibility follow general patterns in soil type and aquifer attributes. It is notable that some variability in levels of susceptibility across individual watersheds is lost due to averaging. This can be seen particularly along the Minnesota River and the beach ridges of the northwest. Also, the susceptibility to contamination is driven largely by subsurface features, often not readily apparent and divergent from the surface watershed boundaries.
This index is directly related to groundwater quality. The geomorphology of each watershed determines the risk level for groundwater contamination. This risk level should be used to inform decisions regarding appropriate land uses in vulnerable locations.
Groundwater and surface water are an interconnected resource. Water quality contamination at any point in the system degrades the ability of water to effectively provide ecosystem services throughout the connected hydrologic cycle.
This indicator is associated with the timing of the transfer between surface water and groundwater. In areas of high vulnerability, the exchange of surface water and ground water is more rapid, which impacts the hydrology of the system in a number of ways, for example, through a more flashy hydrograph.
Groundwater contamination can impact biological communities by directly affecting the health of plants and animals. Chemical and nutrient contaminants can enter lakes or streams through the groundwater and degrade the biological health of those systems.
The index is based specifically on Porcher (1989), described at https://files.dnr.state.mn.us/waters/groundwater_section/mapping/sensitivity/docs/porcher1989.pdf
The approach in this analysis is based generally on the established hydrogeology science and associated observations of groundwater contamination. Basic relationships between precipitation, water balance, surface water residency, and the permeability of surface and sub-surface soils and bedrock were used to estimate susceptibilities (Fetter, 2001).
The connection between surface fertilizer and pesticide applications and groundwater contamination is documented in hundreds of studies over the past several decades, both nationally, and in the upper Midwest. For example, Nolan et al. (2002), found nitrate concentrations in ground water generally increases with higher nitrogen input and higher aquifer vulnerability, and consumers of shallow ground water are more likely to drink high-nitrate water. The median nitrate concentration and percent of wells from which water exceeds the EPA drinking-water standard for nitrate are highest in areas with high nitrogen input and high aquifer vulnerability. As another example, Barbash, (2001), found the commonly applied agricultural chemicals of atrazine, cyanazine, alachlor, and metolachlor were significantly correlated with the amount of agricultural land. Acetochlor, an agricultural herbicide first registered in 1994 for use in the USA, was detected in shallow ground water by 1995, consistent with previous field-scale studies indicating that some pesticides may be detected in ground water within 1 year following application.
There is a well documented relationship between susceptibility of groundwater to contamination and the presence of contamination. Contaminants can be measured directly in the receiving waters and studies have shown higher levels of contamination where there is high susceptibility. This index evaluates the potential for additional degradation of groundwater rather than reporting existing impairments in groundwater quality.
The ranking process is a straightforward area weighted mean for each watershed based on the potential susceptibility to contamination from the model. The average for a watershed may mask smaller areas of very high or very low risk in the aggregated score.
The original model noted that ”if funding is available, refinement of the assessment methodology may include the addition of data and new parameters such as depth to water, thickness of the vadose zone, potential sources of contamination, and other cultural or demographic impacts.” (Porcher, 1989).
The location of and extent of groundwater aquifers relative to surface topography and watershed boundaries could be mapped and combined with information on the specific location and application rates for potential contaminants.
The current development of a County Geologic Atlas across Minnesota provides more detailed information on groundwater resources and susceptibility. When the County Geologic Atlas is completed statewide, more detailed and updated information should be substituted for the current model.