National Water-Quality Assessment (NAWQA) Program
A total of 63 wells in three well networks were sampled in 1988-1993 (Shedlock and others, 1999), and resampled in 2001-2002 (Debrewer and others, 2008). Two aquifers were investigated; the carbonate rock aquifer in the Great Valley (24 wells), and the surficial aquifer on the Delmarva Peninsula (39 wells). There was no significant changes of nitrate concentrations in groundwater from the Great Valley carbonate aquifer, but there was a significant decrease of atrazine, DEA, and prometon concentrations between 1993 and 2002. Such changes are most likely related to nitrogen and pesticide use in the region. Groundwater samples from the carbonate aquifer represent a mixture of older water in the system and a large fraction of young water entering the aquifer through networks of irregular fractures, bedding planes, and solution openings.
Locations of the Great Valley carbonate rock areas of the Potomac River Basin and surficial aquifers on the Delmarva Peninsula (POTO and DLMV) study areas (from Debrewer and others, 2008).
Concentrations of nitrate, metolachlor, prometon, and simazine increased between 1988 and 2001 in the predominately sand and gravel surficial aquifer on the Delmarva Peninsula (Debrewer and others, 2008). Degradates of metolachlor and alachlor were commonly detected; concentrations were typically at least an order of magnitude higher than their parent compounds. Groundwater in this aquifer flows through unconsolidated sediments along more well-defined flow paths with relatively little mixing; observed changes in groundwater quality are more closely related to changes in chemical applications or land use close to the well than in the carbonate setting. Prometon and simazine are commonly used on rights-of ways, and the observed change may be related to regional population growth and urbanization during the period of recharge. Decreasing detections of alachlor and its degradates in groundwater is expected because alachlor use has largely been replaced by acetochlor.