Can Gravel Mining and Water Supply Wells Coexist?
Executive Summary • Introduction • Problem Statement • Objectives • Methods • Results (field observations, lab results, statistical analysis) • Discussion • Acknowledgements • Appendices
Sand and gravel deposits are a legacy of the continental ice sheets that melted more than 10,000 years ago. As the ice melted fast moving rivers formed that left behind deposits of coarse sand and cobbles (called eskers). Where the rivers ran into the sea, large deltas formed with layers of sand and silt. In modern times, the ice is gone, the melt-water rivers have disappeared and sea-level has changed from where it once was. What are left, are scattered deposits of sand and gravel that have become important natural resources. Most people are familiar with the need for sand and gravel for construction material. Fewer people are aware that these same sand and gravel deposits are also prime sources of potable groundwater. Sand and gravel deposits are very porous; great amounts of water can pass easily through this geological material making it a source of large quantities of high quality ground water. Sand and gravel aquifers are very desirable for public and private water supplies.
Sand and gravel deposits and their associated aquifers are resources that cover approximately 5% of the State of Maine (Figure 1). There are competing needs for this same resource: mining for construction material and pumping for drinking water. According to the Maine Department of Environmental Protection (M. Stebbins, pers. com.), there are currently 160 active sand and gravel pits operating with permits. These pits cover areas from five to 260 acres. The pits are distributed unevenly and some towns may have up to 14 active pits within their borders. In addition there are an unknown number of smaller pits that do not require permits since they are smaller than five acres (38 MRSA §490-A). Historically, mining in rural areas was not in conflict with other uses of natural resources. However, changing demographics and development in Maine are bringing more people in contact with sand and gravel mining.
There are over 2,000 public water supply wells in Maine and many thousands of private wells. Many of the highest-yielding wells are constructed in sand and gravel aquifers. All aquifers depend upon rain and melting snow to restore water lost to consumption, or discharge to streams over the course of the year. Changes at the land surface can affect the quantity and quality of water in aquifers. Sand and gravel mining may affect aquifers in a variety of ways. One is the modification of recharge area to groundwater supplies by changing the shape of the land surface such as turning a hill into a flat area, or even a hole. Water no longer flows along its original pathways. Such changes may increase or decrease rainwater recharge to groundwater. So one question that can be asked is: How does mining affect the hydrology of the underlying sand and gravel aquifer?
Another effect of sand and gravel mining is the loss of the protection provided by soil as it filters out pollutants. Removing the highly concentrated organic layer of soil found on the surface of sand and gravel deposits decreases the soil’s ability to bind up substances and thus clean water as it passes through its pores. This loss develops new avenues for contamination to enter groundwater. This type of problem was discovered when old gravel pits, that traditionally were used for dumps, contaminated soil and water in the aquifers. Some sand and gravel aquifers were impacted by dumps to the point that they became unsafe to drink. Current gravel mining regulations are intended to avoid future contamination of groundwater resources, but little is known about how well the rules work. Another question is derived from these concerns: Does sand and gravel mining make the underlying aquifer more vulnerable to contamination?
There are also other environmental issues connected with the reclamation of former gravel pits and inappropriate land-uses at former pits. Gravel pit reclamation, or the lack of reclamation, can have an effect on water quality.
The quality of drinking water has far-reaching repercussions to personal health and other costs to society. The Maine Drinking Water Program completed the Source Water Assessment Program (SWAP) in 2004. They studied public water supplies throughout the state of Maine to determine the risks to sources of supply. Community Supply Wells were given special scrutiny because they are the sole source of drinking water for their customers. A Community Water Supply serves a community, town, trailer park, or some other facility that serves the same group of people on a daily basis. Schools and restaurants are classified differently because people come and go at these locations.
As an example of how water supplies are threatened, Figure 2 contains a summary of risks to community water supply wells compiled during the SWAP process. The risks are grouped into five categories: well type; existing chronic or acute risk; and future chronic or acute risk. Risks are defined in part by how the well was constructed and the thickness of overburden near the well. A deep well with thick overburden (such as a bedrock drilled well through thick soil with many feet of casing) will be at lower risk than a shallower well constructed in a sand and gravel deposit. Very porous overburden, as would be the case for a well constructed in sand and gravel, increases the risk of contamination from surface spills. The other types of risks are related to activities that could affect water quality in general. These risks are split into two sub-groups depending upon whether they exist now or are likely to be a concern in the future. These risk sub-groups have a time factor and are further classified as either chronic or acute. An example of a chronic risk would be the leaching of septic system wastes into the groundwater over long periods of time. An example of an acute risk might be a catastrophic oil tank spill. A key conclusion from this study is that all water supply wells will see increased acute and chronic risks in the future. The cause? Human activities related to land development and increased human presence in the area of supply wells.
Figure 2: The Source Water Assessment Program Study of Community Wells (from the Maine Drinking Water Program, 2004). The column heights equal the number of wells in a category. The colors refer to risk- red is high, yellow is moderate, and blue is low. A key finding of this study is that we anticipate a large number of problems in the future.
In 2002, members of the Lamoine Conservation Commission approached researchers at the Mitchell Center at the University of Maine because they were increasingly concerned about how sand and gravel mining was affecting the local aquifer. Citizens observed the activities associated with development: land being sold; new developments being established; lots getting cleared; and, sand and gravel trucks leaving the town filled with material from the aquifer. Were these activities going to harm the town’s water supply?
Development pressure is evident in many coastal communities. The commission worked with faculty and students at the College of the Atlantic (COA) in 2004 to develop a build-out scenario for the town of Lamoine. This analysis looked at three land-use types in Lamoine: developable; sand and gravel deposits (mining); and conservation land. The class simplified the changes in the demographics by assuming that only 50 percent of the lots were developed (see Figure 3). The development scenario shown is consistent with recent findings that 400% of the United States’ population has moved from inland to coastal properties in the past 20 years (Deidre Mageean, 2005 pers. comm.). This analysis looked only at land uses; water supply availability was not evaluated.
Figure 3: Full build out scenario for Lamoine developed by the College of the Atlantic Land Use Planning Class, Spring 2004.
The growth rate in the town of Lamoine and the results from the COA class project reinforced the commission’s concern about there being ample water resources to supply the growing population and about how much sand and gravel could be removed from the aquifer before it was harmed.
The commission worked with the Mitchell Center to design a project to collect data about gravel pits and groundwater. Funding for the work came from several funding agencies and foundations. The project objectives were as follows:
- Inventory drinking water supplies near active pits to update existing information;
- Inventory drinking water supplies near large reclaimed pits;
- Assess changes in water quality using existing information from selected locations;
- Develop a methodology to assign risk rankings to groundwater resources;
- Assess how well current regulations protect the water resources; and,
- Provide the results of this study to towns, concerned citizens, and regulators to h elp them manage local resources more effectively.
One intended outcome of this study was to provide a context for evaluating how well our natural resources are being protected by state regulations and to provide information to communities to help them manage natural resources. Since the aquifer (sand and gravel deposits) studied extended through the towns of Hancock and Ellsworth, these towns were included in the study (Figure 4).
Figure 4: Location of sand and gravel aquifer (orange and red shape) in Ellsworth, Hancock, and Lamoine, Maine. Public water supply wells are shown as green dots. Map from the Maine Drinking Water Program online mapping program.