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
This study produced answers to the two main questions. The first was – How does mining affect the hydrology of the underlying sand and gravel aquifer? Based on interviews with well owners and observations of surface water features there was no evidence of significant changes in surface or groundwater hydrology. Water level measurements and observations made during the field study can now serve as a reference for future measurements. The absence of significant changes in hydrology is encouraging in that short term disruptions are seemingly rare. Repeated water level measurements in future years will address the question of long-term disruptions.
The second question answered was – Does mining make the underlying aquifer more vulnerable to contamination? Based on the data collected, water quality has been degraded by salt and nitrate. Degradation of water quality occurs in different areas; however directly linking changes in water quality with gravel pit operations goes beyond the limits of the data. There may be an increase in nitrate in surface waters near gravel pits, but the number of samples analyzed is too small to make this a certainty.
One of the questions asked was, how does the water chemistry vary across the aquifer? We have tried to answer this question by plotting the chemistry results on a map (Figure 5). In this figure the major ions (calcium, magnesium, sodium, sulfate, chloride, and carbonate) each form corners of a pyramid. The pyramid is folded down so that we can see each side at once. All of the results for surface water, springs, and groundwater cluster near each other. This indicates that there is some consistency in chemistry across the aquifer. It can also be noted on the figure that the points form an arc that stretches towards the chloride and sodium corners (lower right corners). This is confirmation of the effect of salt on water quality. Lines connect the points on the figure with locations on the map (aerial photograph). Notice that the lines cross over each other. This means that there is not a systematic change in the aquifer in any one direction. More detailed studies are needed to understand why the chemistry changes by location.
Since the results of the water chemistry suggest that salt is affecting water quality, the results were examined to look at the possible connection between sample locations, roads (salt use), and gravel pits (possible storage areas of salts or trucks). A geographical information system (GIS) was used to plot roads, gravel pits, sampling locations, surface water bodies, and chemistry results on a base map (Figure 6). This map shows streams near the aquifer and the surrounding watershed. Each filled red circle on the map marks a site sampled. The size of the circle corresponds to a chloride concentration with the largest circle indicating the largest result. The greatest concentrations of chloride appear to occur near major roadways. Road salting in the winter is a likely source of this chloride. Elevated concentrations near the coast may reflect the influence of the nearby bay.
The ratio of sodium to chloride has also been plotted on the same map. In road salt, the ratio of sodium to chloride is close to one (each occurs in equal concentrations). Seawater should have a sodium to chloride ratio less than one. A sodium to chloride ratio much greater than one may reflect some other type of source or interactions in the subsurface. On the map, bright green and light blue points are those sites most likely affected by road salt. Dark green points mark locations with a potential marine influence and dark blue points are grouped as complex sources. Most of the locations appear to have sodium to chloride ratios indicative of road salt based on this simple analysis.
Gravel pits are defined by shaded areas on Figure 6 and the shapes are fairly accurate representations of actual size and location. An inspection of this map shows that the samples did not cluster around gravel pits. Therefore, there is not a strong spatial relationship between ‘salt-affected’ wells and gravel pits. There also was no statistical association between the distance from a sample point to a gravel pit and chloride concentrations.
The water quality data must be interpreted with care. Chemistry results may change in concentration by location due to seasonal precipitation amounts and transport of substances into ground water. Presently field data indicate that water quality degradation is limited in both magnitude and occurrence location. Further studies will generate more data on groundwater chemistry to demonstrate how water quality changes across the whole aquifer and surrounding towns.
Some of the gravel pits in this study have been in operation for more than eighty years. Unfortunately, there are very few documents or much institutional memory of historical activities. Activities have been inferred from field observations and interviews. Quantifying future impacts on local hydrology will be possible now that some baseline measurements have been made. The baseline water elevation data will be updated on an annual basis to map out changes over longer periods of time.
An added concern that was outside of the project scope was how pits were managed and prepared for disuse. Mining below the water table was noted in at least one pit and maintenance of separation distances above the water table was not always apparent. Old inactive pits were observed to be used for storage of a variety of construction equipment, vehicles, and debris. Some pits were obviously being used as small dumps. Former community landfill sites in gravel pits have been documented to affect water quality in many towns throughout the state. Lamoine continues to experience poor water quality in some wells located near Berry Cove due to an old landfill in the aquifer. Reclamation of inactive pits is essential to prevent degradation of groundwater by illicit and unregulated debris dumping.