Hundreds of millions of years ago, Southeast Minnesota was covered by a warm, shallow sea. Ancient invertebrates dominated these waters and left traces of themselves in these Paleozoic rock formations. Glaciers later swept over the rest of Minnesota but were completely absent from this area. Today, this region of deep valleys, limestone bluffs and forested hills is a place of great beauty. Unfortunately, the ancient limestone and coral reefs that remain are deeply fractured "karst" features. With no glacial overburden, these cracks provide conduits for the rapid movement of contaminated surface water into deeper aquifers.
Nearly all Southeastern Minnesota communities use groundwater as their primary source of drinking water (Porscher, 1989). Typically, communities are utilizing these potentially contaminated "karst" aquifers for their drinking water. It is critically important that these aquifers are protected from contamination sources.
Petroleum contamination remains one of the primary sources of groundwater pollution in the United States (EPA, 1994). The State of Minnesota has over 14,000 petroleum release cleanup sites with almost 250 in Olmsted County and 182 in the city of Rochester (MPCA, 2000). These areas of demonstrated petroleum releases often lie close to community wells and can negatively affect the quality of the water from these wells (Figure 1). Remediation project managers at the Minnesota Pollution Control Agency (MPCA) felt the need for developing a scientific method for evaluating the potential impact of petroleum release sites on community wells. This was especially important in SE Mn given the karst nature of the bedrock, which makes these resource aquifers especially vulnerable. The second objective was to design a method so that small communities unable to afford expensive capture zone delineations could still develop wellhead plans to protect their water supplies. The method described in this paper has now been successfully used in a variety of geologic settings and in communities of varying size.
This study consisted of creating a model that considered the interaction between five separate modules. See Table 1.
Figure 1. Petroleum release sites (circles) and community wells (triangles) in Rochester, MN
The values used in this study were arrived at through consideration of previous research and by discussions with hydrogeologists, MPCA project managers, and scientists at the Minnesota Department of Health (MDH) and the United States Geological Survey (USGS). These values will be taken up in greater detail in the evaluation of each module.
| Module Abbreviation | Module Name | Range of Factors |
| P | Petroleum release site Proximity to Community Wells | 1 - 58 |
| GW | Influence of Groundwater Flow Direction | 1 - 10 |
| PS | Pollution Sensitivity of petroleum release site location | 0.25 - 40 |
| CW | Community Well Contamination Potential | 1 - 10 |
| LS | Petroleum release site Contamination Potential | 0.045 - 105 |
Analysis primarily involved creating individual modules and then combining them to gain useful information in the following sequence.
1) Gather all available data including roads, surface water bodies, county boundaries, geological coverages, parcel data, petroleum release sites, and well locations.
2) Buffer all wells to a distance of 3200 meters in increments of 100 meters. Record all distances from community wells to petroleum release sites and convert these distances into proximity values (P).
3) Digitize a groundwater flow direction (GW) map of the study area and use it to determine the (GW) relationship between each petroleum release site and community well. Convert groundwater flow directions into values.
4) Buffer petroleum release sites to 100 meters. Intersect this buffer coverage with the pollution sensitivity (PS) map to obtain a (PS) factor.
5) Evaluate all wells and assign a risk factor. Convert this risk factor into a community well value (CW).
6) Evaluate every petroleum release site for risk and assign a risk factor to create the petroleum release site value (LS).
7) Analyze the relationship between proximity and groundwater flow for each petroleum release site and community well and assign a value (P-GW). Sum these values for all petroleum release sites within 3200 meters of a given well.
8) Analyze the relationship between the (P-GW) value for each petroleum release site and the pollution sensitivity rating (PS) and assign a value (P-GW-PS). Sum these values for all petroleum release sites within 3200 meters of a given well.
9) Analyze the relationship between the (P-GW-PS) value for each petroleum release site and the community well factor (CW) and assign a value (P-GW-PS-CW). Sum these values for all petroleum release sites within 3200 meters of a given well.
10) Analyze the relationship between the (P-GW-PS-CW) value for each petroleum release site and the petroleum release site contamination factor (LS) and assign a value (P-GW-PS-CW-LS). Sum these values for all petroleum release sites within 3200 meters of a given well.
11) Create views and display information and values on maps.
12) Create map layouts of these and other views and print them.
Each of the modules was considered separately and sequentially in the order mentioned above (Steps 2 - 6). Modules were broken down into components and each component was assessed for how it affected the value of the individual module. The goal of this process was to balance the many influences that define the probability of contamination.
Geographical Information Systems (GIS) provides a means of evaluating and manipulating different factors in order to obtain new information. This study combines five separate modules. Certain combinations provide extremely useful information and these descriptions are provided in the Steps VI-X in the following section.
This model should be useful to persons involved in a wide variety of groundwater protection activities. This section summarizes how individual modules and different combinations can be used.
Identifies locations in a community where petroleum release sites are present. Areas of high density should also identify where commercial and industrial point sources are concentrated.
Regardless of using this full analysis, a digitized groundwater flow direction map is extremely useful for Emergency Response, community planning, and petroleum release site risk management.
This module is most useful in combination with the proximity and groundwater flow direction modules. Without doing this entire analysis, knowing the pollution sensitivity of the location of a petroleum release site should be helpful in assessing site risk.
Very similar to one already created by the MDH; however, this module can help to assess which wells are at greater risk for contamination.
Helps the MPCA project manager look at a site and get an immediate assessment of the risk the site poses.
Identifies areas of a community in which petroleum release sites and point sources are more likely to present problems.
This combination should be able to clearly define areas within the P-GW matrix from Step VI above that are particularly problematic. This module could be useful in guiding zoning considerations that will minimize the possibility of impacts to community wells.
This module gives us a current snapshot of the community based on the present configuration of community wells. This module can help recognize which community wells are inherently more risky. It can guide pumping rates, shutting down of old wells and general well management. It is also an extremely useful guide for Emergency Response situations in determining if a spill might pose a greater than average risk of contaminating community wells.
Identifies the areas of a community that are particularly vulnerable to groundwater contamination due to petroleum sources. This module can be used to guide new well placement.
This combination provides an overall assessment of which community wells are at greatest risk for petroleum contamination as well as which petroleum release sites are most at risk for causing these problems. These considerations can guide well pumping rates, well closure, and petroleum release site clean-up prioritization.
This model closely mimics more complex drawdown area delineations. Three of the factors that define this model also dominate inputs to drawdown delineations. These factors are: groundwater flow direction, pumping rates and pollution sensitivity. Communities that have already delineated their drawdown areas can utilize this study in several different ways. First, they can use their drawdown areas to clip out all petroleum release sites falling outside their range. Second, they can retain all petroleum release sites in their study area and create a new GIS layer that increases the probability of contamination within those areas. Finally, well pumping test data used for delineations provides invaluable information for using the most accurate groundwater flow direction values.
One of the weaknesses of the current wellhead protection efforts is that they do not analytically recognize the difference between contamination point sources. Petroleum release sites that have had a catastrophic tank collapse should not be treated the same as sites where reportable, but minimal, contamination was found. This model addresses that important missing component.
This model can also be used to define a variety of other environmental point source problems. Analytical methods have been developed for looking at superfund sites, hazardous sites, feedlots, etc. Additionally, tank sites as well as hazardous waste generator locations can also help a community define where source problems are likely to occur in the future. Length constraints on this paper do not permit us to describe the analytical methods used for these additional sources.
The system of evaluation described in this study makes it easy to understand which areas of a community are the most vulnerable to contamination. The next step in this assessment involves comparing these values to other communities. A relative scale was developed in order to assist communities in comparing their well values and contamination potential with other municipalities.
This paper outlines a method of assessing the relationship between petroleum release sites and community wells in Rochester, MN. It offers a variety of ways of looking at five modules: petroleum release site proximity, groundwater flow direction, pollution sensitivity, community well contamination potential and petroleum release site contamination potential. Each of these modules was given a representative sphere of influence. Modules were then combined to obtain different information.
This study found that community wells in the central and southern parts of the city were at the greatest risk for experiencing petroleum contamination. These areas were identified because of the combination of petroleum release sites near community wells, the groundwater flow direction in these relationships, the generally sandy soils at these petroleum release sites, high ratings for community well problems as well as the riskier nature of the petroleum release sites themselves.
This study should be able to assist MPCA staff managing petroleum release site cleanups and community well managers, as well as guiding the city of Rochester in some of its land use and zoning. Professionals can be guided in their decision making by understanding how information can be obtained by different combinations of modules.
This type of study can be easily completed in most communities. Smaller communities with few petroleum release sites and municipal wells would require minimal effort while larger cities require more time.
One must act cautiously before drawing unwarranted conclusions from this study. It would be easy to conclude that serious problems are present, even if they are not. This study provides an assessment of greatest risk; however, it does not imply that petroleum contamination has or will occur in a given location. For instance, in the city of Rochester, no petroleum volatile organic compounds have ever been found in community wells that exceeded the Health Risk Limits recommended by the MDH (RPU, 2000). This study is an attempt at helping communities like Rochester make sure they never will.
Environmental Protection Agency.1994. National Water Quality Inventory, EPA 84-R-94-001, US EPA, Washington, D.C, 450 pp.
Minnesota Pollution Control Agency. 2000. Leaksite information gained from Tales database, St. Paul, MN.
Porcher, Eric. 1989. Ground Water Contamination Susceptibility in Minnesota. Minnesota Pollution Control Agency, St. Paul, MN. 29 pp.
Rochester Public Utilities. 2000. Personal correspondence with Bill Cook. Rochester, MN