Final Report
June 2006

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

Laboratory Results

Laboratory testing results and field temperature measurements are summarized in Table I. These results provide a general assessment of water quality in the sand and gravel aquifer. Concentrations for some results are reported in micro-equivalents per liter (μeq/L). Equivalents measure moles of charge and in this case, the micro-equivalent value is equal to micro-mole quantity times its ionic charge. The results are compared to any state or federal standard for drinking water quality.

Temperature values ranged from 1º to 9ºC (34º to 48ºF) in springs, streams, and ponds. The water temperature is related to the depth from which the water originates. Deep groundwater tends to be at a constant temperature close to 10ºC (50ºF) year round. Using temperature as an indicator, these springs appear to be fed by deeper groundwater: Latona, Cold Spring Water Company, Boat Shop, and Washington Junction. The other springs were cooler and either are fed from shallower sources, mix with colder water near the surface, or flow slowly enough to be cooled down.

The pH of the samples ranged from 4.73 to 6.94. All of the springs had pH values within 0.5 units of 6. This range is normal for ground waters in the region. Blunts Pond had the lowest pH value. It is not unusual to see surface waters with lower pH values because organic acids and atmospheric inputs affect surface waters, but not ground waters. The drinking water standard for pH is between 6.5 and 8.5 pH units.

Conductivity is a measure of how much ionic material is dissolved in the water. It provides a simple check on the accuracy of the laboratory analyses of major ions. The water samples had conductivities ranging between 18 and 300 micro-siemens per centimeter (μS/cm). These values are reasonable for groundwater in Maine. In comparison, pure distilled water would be <1 μS/cm and seawater exceeds 10,000 μS/cm. The drinking water standard is based on total dissolved solids and the equivalent conductivity is in the range of 500 to 700 μS/cm.

Alkalinity is a measure of how well the solution can neutralize acid. In groundwater, this is closely related to the amount of base cations (calcium, magnesium, sodium, and potassium) and bicarbonate ion. The alkalinity measured ranged from -17 to 303 micro-equivalents per liter (μeq/L). Blunts Pond is actually slightly acidic and it has no acid neutralizing capacity, hence it has a negative alkalinity. Most of the springs had values near to 200 μeq/L. There is no drinking water standard for alkalinity.

Dissolved organic carbon (DOC) is the amount of organic matter that passes through a 0.45 micrometer (μm) filter. Dissolved organic carbon in surface waters can come from decaying plants and animals. Groundwater usually contains very low concentrations of DOC because of a combination of soil organisms that consume carbon and the filtering effect of soil. Elevated concentrations of DOC in groundwater may indicate pollution from septic systems, or it may indicate that the groundwater has a close connection with surface recharge (poor filtering). Measured DOC values ranged from 0.3 to 12.6 milligrams per liter (mg/L). The deep springs and other areas of ‘pristine’ groundwater had DOC concentrations <1 mg/L. There is no drinking water standard for dissolved organic carbon.

Calcium and magnesium are usually derived from rocks through chemical weathering. The concentrations in groundwater may reflect in part how long water has been in contact with rock, as well as the solubility of the rock. Calcium was detected in the range of 31 to 577 μeq/L (approximately 0.63 to 12 mg/L). Magnesium was detected in the range of 31 to 200 μeq/L (approximately 0.38 to 2.43 mg/L). The springs had similar concentration ranges for calcium (~200 μeq/L) and magnesium (150-200 μeq/L). There is no drinking water standard for calcium or magnesium. These elements do contribute to hardness, and when the total exceeds 150 mg/L it is general considered undesirable.

Sodium is derived from rocks through chemical weathering. It can also enter into water from road salt use and the effects of being near to the ocean (known as the sea-salt effect). In natural geological settings, sodium is found in smaller concentrations than calcium. Whenever, sodium concentrations exceed calcium, some form of salt contamination is suspected. Sodium was detected in the range of 65 to 2,043 μeq/L (1.5 to 47 mg/L). Sodium concentrations were generally greater than calcium in all samples, suggesting a widespread salt effect. Even the pristine springs had sodium concentrations >100 μeq/L (>2.3 mg/L), and some exhibited unsafe concentrations of sodium. The maximum concentration for sodium in drinking water in Maine is 20,000 ppb (870 μeq/L).

Chloride is rare in rocks in Maine because it is very soluble as a mineral. Chloride compounds are not stable in our wet climate and they quickly dissolve. In this setting, chloride can only come from road salt, sea salt, and household septic systems. Chloride was detected in the range of 73 to 2,177 μeq/L (approximately 2.6 to 77 mg/L). The pristine springs all contained detectable concentrations of chloride. Elevated concentrations of chloride matched sodium, strongly suggesting local salt contamination. The maximum concentration for chloride in drinking water in Maine is 250,000 ppb (7,052 μeq/L).

Nitrate is generally found in no more than trace concentrations in groundwater. This is because nitrate is rapidly consumed by organisms in the soil. Excess nitrate in groundwater can almost always be connected to agriculture or waste disposal (e.g. septic systems). Nitrate was detected in the range of 0.1 to 342 μeq/L (approximately 0.01 to 21 mg/L). The safe limit for drinking water was exceeded in two spring samples (Boat Shop and Washington Junction) and approached in another (Cold Spring Water Company). The maximum concentration for nitrate in drinking water in Maine is 10,000 ppb (161 μeq/L).

Sulfate is generally found in trace concentrations in groundwater in most regions of Maine. Sulfate can come from natural sources in rock, from contamination such as landfills, and even seawater. Sulfate was detected in the range of 39 to 250 μeq/L (approximately 1.9 to 12 mg/L). Most of the springs had sulfate at concentrations <100 μeq/L. The maximum concentration for sulfate in drinking water in Maine is 250,000 ppb (5,207 μeq/L).

In general, the chemistry of surface and ground waters falls within acceptable ranges for most analytes tested except for sodium, chloride, and nitrate. Two wells, three springs, and one brook sample exceeded the maximum acceptable concentration for sodium in drinking water. Two springs exceeded the maximum acceptable concentration for nitrate in drinking water. Nitrate, in a concentration below the maximum limit, was detected in the Cold Spring Water Company spring. The high sodium concentrations detected, along with elevated concentrations of chloride in the same samples, are indicators of salt contamination. The salt and nitrate detections are items of concern and the groundwater should be monitored for evidence of increases with time. The results of water levels measurements are presented in Appendix B. These measures are intended to be referenced in future studies of water levels. Longer periods of time are needed to determine if there are systematic changes in the vertical location of the water table.

TABLE I. Summary of Laboratory Results for Water Analysis. Underlined values exceed safe drinking water limits.

TABLE I continued. Summary of Laboratory Results for Water Analysis. Underlined values exceed safe drinking water limits.

Statistical Associations

The results of the chemical analyses were tested using the Kruskal-Wallace method to determine if samples collected near gravel pits were significantly different from other samples. The chemistry results were separated into two categories for this analysis: surface water (pond or stream) and groundwater (well or spring). Only one chemical parameter, nitrate, was found to exhibit statistical significance with closeness to gravel pits (p<0.1). Nitrate concentrations were greater in surface water near gravel pits. Caution is needed in understanding the meaning of this difference because of the small number of samples analyzed. However, this effect should be re-examined with any future water sampling program.

The results of the Pearson correlation analysis give an indication of how different water chemistry variables behave. The results for associations showing correlations with r>0.5 are presented in Table II. These associations provide clues about how the water chemistry varies as pairs (i.e. two variables are compared at a time). Calcium and magnesium exhibited strong correlations with pH, so increasing these two chemical variables was related to an increase of pH. Most of the major ions correlated with conductivity; sodium and chloride (salt) had the most pronounced associations with r>0.9. Calcium, magnesium, and potassium all showed some correlation with each other which suggests that they occur together. Interestingly, nitrate and sulfate also correlated with these same three elements while sodium did not. This suggests that calcium, magnesium, potassium, nitrate, and sulfate have a common source in the aquifer. The different associations determined for sodium, and especially the very strong correlation between sodium and chloride, reinforce the notion that these chemical variables are coming from a different source (i.e. salt).

TABLE II. Pearson correlation coefficients for chemical parameters.