Background

Lead concentrations from water samples collected from some wells in the Grafton Ridge community in Harford County exceed the U.S. Environmental Protection Agency’s Action Level of 15 micrograms per liter (ug/L). The source of the lead in the water has been a matter of dispute. Elevated lead concentrations are uncommon in studies in Maryland counties with similar hydrogeologic conditions (Bolton, 1998; Dine and others, 1995). A comprehensive study of groundwater quality in Baltimore County had only a few lead concentrations above 15 u/L, and may have been due to acidic water flowing through older plumbing fixtures that contained lead solder, rather than having a geological source (Bolton, 1998).

The portion of Harford County located north and west of Interstate 95 is underlain by crystalline igneous and metamorphic rocks, and is part of the Piedmont physiographic province of Maryland. South and east of Interstate 95, the county is underlain by younger unconsolidated Coastal Plain deposits. Groundwater in the Piedmont is typically acidic (pH less than 7). This is due primarily to the relatively low reactivity of quartz, feldspars, and other igneous minerals. When groundwater flows through rocks containing relatively soluble minerals (such as limestone, or sandstones whose grains are cemented together with soluble minerals), acidity is consumed as the minerals react with the aggressive water, and pH increases accordingly. If an aquifer contains accessible lead, the acidic water may leach the lead from the geologic formation. This has not been observed to be a major problem in Baltimore and Howard Counties (counties in similar terrain that have been most thoroughly investigated). However, pH and lead concentrations have not been systematically investigated in Harford County. Although some of the aquifers in Harford County are similar to those in Baltimore County and might be expected to have similar ranges of pH and lead, there can be local variations in rock geochemistry that can cause variations over short distances. Furthermore (and more significantly), much of the county is underlain by rocks which have not been extensively tested, particularly the gneissic and mafic units (including the Port Deposit gneiss, Baltimore Gabbro, and ultramafic and gabbroic rocks).

Study Objectives

The Maryland Geological Survey will take part in a study, in partnership with the Harford County Health Department (HCHD) and the Maryland Department of Health and Mental Hygiene (DHMH), in which approximately 80 water wells will be sampled and tested for lead, pH, chloride, and specific conductance. The water-quality data will be evaluated in relation to geologic formations, and will provide the County with a baseline of data that can be used to guide management initiatives. The HCHD plans to collect data on the plumbing system at each site, and this data will be used to help identify the sources of lead found in drinking-water systems.

Methods of investigation

1. Working with the HCHD, MGS will identify geologic regions of the Harford County Piedmont where wells should be identified for sampling. Approximately 80 wells will be sampled. Identification of potential wells for sampling will be conducted according to the following guidelines:

• Wells will be restricted to those completed in bedrock (no wells in unconsolidated Coastal Plain deposits).
• Wells will be distributed proportionally among the major geologic groups in Harford County.
• About half of wells will be in the Wissahickon Formation (upper and lower politic schists; metagraywacke, metaconglomerate, boulder gneiss). These rocks underlie the northern and northwestern part of Harford County, and comprise approximately half of the county. The remaining half of the wells will be divided between the various ultramafic rocks (quartz gabbro and quartz diorite gneiss; Baltimore Gabbro, metagabbro and amphibolite, and ultramafic and gabbroic rocks) and other intrusive rocks (Port Deposit gneiss, James Run gneiss, muscovite quartz monzonite gneiss, Baltimore Gneiss, and others).

2. MGS will obtain well and site information from HCHD for all identified wells; assign well numbers; code and enter wells into database (tentative USGS).

3. MGS will collect samples from all selected wells. pH, specific conductance, and chloride will be analyzed in the field; water samples will be collected for lead analysis by DHMH.

4. MGS will evaluate all collected water-quality data with respect to geology and other relevant factors.

5. MGS will write and publish an interpretive report documenting all aspects of the project. Depending on the significance and amount of content, the report will either be an MGS Report of Investigations or and MGS Open-File Report.

References

Bolton, D.W., Ground-Water Quality in the Piedmont Region of Baltimore County, Maryland: Maryland Geological Survey Report of Investigations No. 66, 1998.