Residential Septic System Impacts on Groundwater Quality in Maine
CHARACTERIZATION OF NITRATE CONCENTRATIONS IN DOMESTIC WELLS AT 18 SUBDIVISIONS
By:
Steven R. Pinette and William T. Noble
Maine Department of Environmental Protection
Daniel B. Locke and Marc C. Loiselle
Maine Geological Survey
This study was funded in part by a nonpoint source pollution grant
from the U.S. Environmental Protection Agency
under Section 319 of the Clean Water Act
June 1999
Maine Department of Environmental Protection
Bureau of Land and Water Quality
Division of Environmental Assessment
Environmental Geology Section
Document No. DEPLW1999-7
EXECUTIVE SUMMARY
The Maine Department of Environmental Protection and the Maine Geological Survey ex-amined nitrate-nitrogen (NO3-N) concentrations in domestic wells at 18 unsewered subdivisions (using individual septic systems instead of a public sewer system) to determine the possible im-pact of on-site septic systems on ground water quality. Samples from 470 wells were analyzed to provide a one-time "snapshot" of NO3-N levels in the ground water. In addition, samples from 59 wells at four of the subdivisions were tested quarterly to evaluate any changes in NO3-N con-centration over time.
The subdivisions selected for well water sampling represented several development ages, geologic settings, and housing densities, and had at least 20 or 30 existing homes. In addition, subdivisions were selected where ground water beneath the developments was not affected by other NO3-N-contributing activities, such as fertilized cropland, or land used for manure dispos-al or spreading.
Nitrate-nitrogen concentrations in the wells were reviewed in relation to site geology (or soil type), development density, and well characteristics, such as type, depth, location, and yield. The broad goals of the study were to assess the effectiveness of the Maine Subsurface Waste-water Disposal Rules (DHS-HE, 1974 and later versions) in protecting domestic well water qual-ity, and to identify site factors that exert the greatest influence on groundwater quality.
Non-parametric statistical testing was applied to the NO3-N analysis results. Findings of the study are summarized as follows:
"Snapshot" domestic well nitrate concentrations -- Analysis results from 470 wells were non-normally distributed, and skewed toward the lower concentrations. Levels of NO3-N measured during this one-time sampling event ranged from 0.00 to 26.52 mg/L. Two wells (< 0.5 percent) had concentrations in excess of the 10 mg/L national primary drinking water standard (PDWS) for NO3-N. The concentration at 22 percent of the wells was recorded at 0.00 mg/L (laboratory minimum detection limit of 0.01 mg/L), and these wells were considered to be tapping ground water unaffected by any sources of NO3-N. Statistical testing revealed that there were significant differences in NO3-N concentrations among the study subdivisions.
Nitrate concentration and well type -- Four well types were identified at the study subdivis-ions: drilled into bedrock, drilled into overburden (soil), driven wellpoints, and dug wells. Sta-tistical analysis showed that there was no significant difference in NO3-N concentration among the four well types.
Nitrate concentration and well depth -- Well depths for 265 wells reporting this information ranged from 9 to 700 feet. Wells with depths of less than 100 feet had higher average NO3-N concentrations than deeper wells. Statistical testing revealed that wells in the depth range of 76 to 100 feet had statistically-greater NO3-N concentrations compared to those wells deeper than 100 feet.
Nitrate concentration and length of well casing -- Casing lengths for 73 reporting wells ran-ged from 0 to 180 feet. Statistical analysis showed a significant negative correlation between casing length and the NO3-N concentration (i.e. longer well casing was associated with a lower NO3-N concentration).
Nitrate concentration and well yield -- Well yields for 126 reporting wells of all types were in the range of 1 to 100 gallons-per-minute (gpm). A statistically significant correlation between well yield and NO3-N concentration was not found.
Nitrate concentration and soil type -- Twenty-eight different soil series were identified at the study subdivisions. These were placed into four general soil groups: thick glacial till, thin till over bedrock (less than 4 feet deep), marine/lacustrine (silt and clay) deposits, and stratified drift (sand and gravel) deposits. Statistical testing revealed that NO3-N concentrations were statistic-ally greater in wells surrounded by thick glacial till compared to those in thin glacial till and stra-tified drift deposits. While results are consistent with expected NO3-N concentrations according to soil type and associated recharge rates, the researchers suggest that other hydrogeologic fac-tors, particularly position of the subdivision in the regional watershed, may be masking the real effect of soil type on NO3-N concentration. Other suggested factors include faulty well or septic system installations.
Nitrate concentration and well proximity to septic systems -- The number of septic system leachfields within 300 feet of each well tested during the snapshot phase, and the topographic position of each leachfield relative to those wells (i.e., upslope, level, and downslope), was de-termined. Statistical testing showed a low, but significant correlation between NO3-N concentra-tion and the following (in decreasing order of significance): number of leachfields within 300 feet in the upslope direction from a well; number of leachfields within 100 feet in the downslope direction from a well; and number of leachfields within a 300-foot radius from a well.
Nitrate concentration and septic system age -- Ages reported for 407 septic systems on the same lot as the tested snapshot wells ranged from 1 to 26 years (at the time the study was con-ducted). Eighty-five percent (348 systems) were 15 years old or younger, and were considered to have been constructed according to the Maine Subsurface Wastewater Disposal Rules (1974 and later versions). Statistical results generally showed that wells paired with septic systems 15 years old or younger had lower NO3-N concentrations compared to wells paired with older systems in-stalled according to pre-1974 siting, design, and construction standards.
Nitrate concentration and fertilizer use -- Lawn fertilizer application information was report-ed for 405 lots in the study subdivisions. Questionnaire results showed that 214 homeowners fertilized their lawns and 191 did not. Statistical analysis found no significant difference in the NO3-N concentration between wells located on lots that were fertilized and those that were not.
Nitrate concentration and temporal variations -- No apparent seasonal pattern of low or high NO3-N concentrations was noted in the monitoring results from the 59 wells tested quarterly at four of the study subdivisions. Statistical testing detected no significant differences in concen-trations among the monitoring rounds.
Conclusions/recommendations -- The study concludes that the Maine Subsurface Wastewater Disposal Rules are adequately protecting residential wells from NO3-N contamination caused by conventional septic systems. To further protect well water quality from potential septic system contamination, the following are recommended:
· Bedrock wells should be drilled relatively deep (to depths of more than 100 feet) and be installed with long casings set and grouted into the bedrock, particularly in areas where bedrock is relatively shallow.
· Wells should be located as far upgradient from septic systems as is practicable (and from other souces of NO3-N).
· A licensing program for septic system installers should be established by the State of Maine.
Recommendations for further study of NO3-N impacts to ground water quality at residential subdivisions include a more-detailed hydrogeologic analysis of a subset of the study subdivis-ions, including installation of long-term monitoring wells. Additional work should focus on how ground water flow systems within the regional watershed affect ground water quality in the sub-divisions, especially considering the effect of subdivision position on the landscape with respect to local and regional ground water recharge/discharge boundaries.
This study is presented in two parts, as separate volumes. Part I includes background infor-mation on environmental and health concerns of NO3-N, a summary of selected related research papers by others, and discussion of the analytical methods, results, and conclusions of this study. Part II presents site-specific information and well data for each of the 18 subdivisions studied, and is intended to provide environmental and groundwater professionals with sufficiently-detail-ed data on the subdivisions so they can develop their own insights about site hydrogeological set-tings and factors that potentially affect groundwater quality.
