Groundwater geochemistry and calcite cementation of the aquia aquifer in southern Maryland

Abstract

The Aquia aquifer in southern Maryland exhibits three regions that are characterized by distinctly different water composition. Region I is about 26 mi (42 km) wide and is parallel to the outcrop area. Water in region I has relatively high calcium (∼30 mg/l), magnesium (∼10 mg/l), and bicarbonate (∼150 mg/l) concentrations and relatively low sodium (∼2 mg/l) concentrations. Region II, parallel to and downgradient from region I, is about 24 mi (38 km) wide. Water in region II is characterized by constant bicarbonate, increasing sodium, decreasing calcium, and decreasing magnesium concentrations. Region III is downgradient from region II and is characterized by low calcium (∼2 mg/l), low magnesium (∼1 mg/l), high bicarbonate (∼300 mg/l), and high sodium (∼100 mg/l) concentrations. The major lithologic constituents of the Aquia aquifer are quartz sand (∼55%), glauconite (∼30%), and carbonate shell material (∼8%). Heavy minerals, clay minerals, and finely disseminated lignite make up about 2% of the aquifer material. Calcite cementation is commonly observed in and near the outcrop area of the Aquia. Stoichiometric chemical models were constructed based on this lithology to describe the water chemistry of each region. These models are (region I) (Ca1−x, Mgx)CO3(s) + CO2(g) + H2O(1) ⇌ (1 − x)Ca(aq)2+ + xMg(aq)2+ + 2HCO3(aq)−, (region II) (1 − x)Ca(aq)2+ + xMg(aq)2+ + Na2 · glauconite(ad) ⇌ Ca(1−x)Mgx · glauconite(ad) + 2Na(aq)+, and (region III) (Ca(1−x)Mgx)CO3(s) + CO2(g)) + H2O(1)) + Na2 · glauconite(ad) ⇌ 2Na(aq)+ + 2HCO3(aq) +Ca(1−x)Mgx · glauconite(ad). The applicability of these models was verified by comparing predicted and observed molar ratios of dissolved species using linear regression techniques. In all cases, close matches were found between predicted and measured curves. Mass balance calculations using the verified chemical models predict a spatial distribution of calcite cementation that closely matches the observed distribution in the Aquia aquifer. This suggests that the observed calcite cementation is postdepositional in origin and has resulted from reaction of groundwater with aquifer material.