Reports
Water-quality data from network wells used to monitor brackish-water intrusion of the Aquia aquifer, Kent Island, Queens Anne's County, Maryland: Data collected through 2013
2014, Drummond, D.D.
Administrative Report 14-02-03
Abstract
The Maryland Geological Survey is currently monitoring a network of domestic wells and observation wells on Kent Island in Queen Anne’s County, Maryland. The purpose of the monitoring program is to determine trends in chloride concentrations and specific conductivities in ground water from the Aquia aquifer associated with brackish-water intrusion from the Chesapeake Bay. Drummond (1988) described brackish-water intrusion on Kent Island, and included discussions of distribution, source, and changes with time of brackish water.
A monitoring network was established in 1986 based on wells sampled in Drummond (1988) and has been sampled annually or semiannually since then. This report presents data from the monitoring program. Table 1 provides a summary of chloride concentrations from selected residential wells for quick reference. Table 2 lists site and construction data for wells in the network. Table 3 lists data for pH, specific conductance, temperature, and dissolved chloride, and dissolved bromide of water collected from network wells from 1982 through 2013. Graphs showing changes in specific conductance and chloride concentration are shown for each well in the monitoring network that was sampled in 2013. Regression lines are also shown for chloride concentrations for wells with more than 10 samples. Field chloride measurements are shown where laboratory measurements are unavailable, but were not used in computing the regression lines.
Thirteen observation and 24 residential/commercial wells in the monitoring network were sampled in August and September 2013. Maps showing well locations are found on figures 1 and 2. The maps also show zones of generalized trends in chloride concentrations of water from network wells.
Most trends in chloride concentrations reported in recent years continued through 2013. Chloride concentrations in some wells in the Bay City/Matapeake Estates areas continue to show an overall increase. Changes in chloride concentrations were generally not seen in samples from the lower Aquia aquifer, where salinities are much higher and screened intervals are farther below the fresh-water/brackish-water interface. An exception to this is well QA Eb 156, which has shown a general increase in chloride since 1998.
Well QA Eb 155 was overtopped by the storm surge of Tropical Storm Isabel in September 2003, and brackish water entered the well bore. This well was not sampled in 2004, 2006 and 2007. In September 2005 the chloride concentration (366 mg/L) greatly exceeded values from previous years. From 2009 to 2011 the chloride concentration decreased back to previous levels as the slug of brackish water gradually dissipated in the aquifer.
The monitoring data indicate that, in general, chloride concentrations are elevated and increasing in the upper Aquia aquifer in the central part of the bay shore on Kent Island; north and south of that central area along the bay shore, chloride concentrations are elevated but do not show a general trend. The lower Aquia aquifer is brackish along the entire bay shore. Inland from the bay shore (about ¼ mile) the entire section of the Aquia is fresh, and does not show evidence of an increasing trend. Increasing trends in chloride concentrations may indicate slight landward movement of the brackish-water interface, but variations mask the trends on parts of Kent Island. The possible explanations given by Drummond (2001) for data through 1999 are still plausible for data through 2013.
