NOTICE OF MEETING
Announcement from the DC-Maryland-Virginia Chapter of the Association of Environmental & Engineering Geologists
DATE: Wednesday 10/19/2016
TIME: 6:00 PM - 6:30 PM - Social & Check-In
6:30 PM - 7:00 PM - Dinner
7:00 PM - 8:00 pm - Chapter Business & Presentation
LOCATION: Brewer's Alley, 124 N Market St, Frederick MD
COST: (dinner and meeting)
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RESERVATIONS: To reserve a seat, please visit our website & E-Pay System at: www.aeg-bwh.org or email Katelyn Vieten by Tuesday, October 18, 2016 at firstname.lastname@example.org
TOPIC: NITRATES IN RURAL GROUNDWATER SUPPLY WELLS NOT FROM FARMING: AN UNEXPECTED FINDING BASED ON FORENSIC HYDROGEOLOGY
PRESENTER: Mr. MARK W. EISNER, P.G., PRESIDENT, ADVANCED LAND AND WATER, INC.
Sharptown is located in an agriculturally dominated portion of the Delmarva Peninsula and is supplied by four wells. Groundwater quality compliance samples have reflected elevated nitrate concentrations for years. Non-point agricultural sources were thought to be the source of the nitrates, but detailed study as a component of an updated Source Water Protection Plan assessment now has revealed a differing and surprising interpretation. Two of Sharptown’s four wells are screened in the unconfined Columbia Aquifer and one is screened in the confined Frederica Aquifer. A fourth well was installed back in 1936 and has a reported completion depth suggesting that it too is screened in the Frederica Aquifer. Additionally, this old well might also be open to the Columbia Aquifer, either through a lack of adequate grout or the presence of an additional screen. As a component of our source water assessment work for the Town, we analyzed available water quality data via End-Member Mixing Analysis. EMMA is a common surface water investigative technique; its use to interpret groundwater contaminant sources and contributions is less common. We used chloride and nitrate as tracers, based on the limited compliance data available. The Columbia and Frederica aquifers differ in their concentrations of these two constituents. Data from the only well confirmed to be screened exclusively in the Frederica Aquifer represented one end-member. One of the unconfined Columbia Aquifer wells was selected to represent the other end-member, given its nitrate and chloride concentrations being similar to those published in available literature.
As expected, we found that the water quality from the old well plotted close to the mixing line between the two end-members, suggesting its water comes from both aquifers. Surprisingly, the EMMA also revealed that a previously unknown, probable point source of nitrate contamination of the local Columbia Aquifer. This interpretation was based on sharply disparate nitrate concentrations, and nitrate-to-chloride ratios, between two closely-spaced and similarly constructed Columbia Aquifer wells. This suggested that the variable-concentration in one of the wells is affected by a seasonally-operative and proximal (to that one well) nitrate point source. The lack of known septic systems near the wellfield, seasonality of nitrate concentration fluctuations and historic reports of sewer system leaks suggest that the nitrate in this one well may originate more from a local sewer system leak than from historic and regional farming. Our review of hydrograph data from a nearby long-term monitoring well supported this interpretation as well. Work is ongoing. and we are seeking to confirm this theory by sampling for Methylene Blue Active Surfactants (MBAS), an indicator of detergents and thus, of domestic sewage. MBAS is synthetic and any detectable presence would indicate a proximal point-source of detergent-laden wastewater. Preliminary analyses indicate detectable MBAS in some of the wells, but in differing locations than we theorized. Sampling and/or laboratory error seems possible, and confirmatory tests are to be conducted. If the sewer system leak suggested by these findings is confirmed, its repair may offer a more economical means of nitrate reduction than treatment and a more assured means than encouraging changes in long-standing local agricultural practices.