Assessment of stream geochemistry in west central Nicaragua during baseflow conditions. (December 2015) - Carey, Anne E.
Abstract: Water samples were collected from streams in west-central Nicaragua at the end of the dry season as part of a study of riverine solutes in volcanic terrains. Streams in volcanic watersheds had lower mean Ca 2+ and Mg 2+ concentrations (0.29 mM Ca 2+ ; 0.13 mM Mg 2+ ; 0.96 mM Ca 2+ and 0.51 mM Mg 2+ ) and slightly higher Ca:Mg molar ratios (2.1) than streams in marine carbonate rocks (1.89). Dissolved Si concentrations averaged 1.12 mM L −1 in the 15 streams sampled, indicating a very high degree of silicate mineral weathering. Younger rocks yielded higher concentrations. At least a portion of the weathering in some watersheds is by strong acids derived from volcanic input. Mean Si, Ca, and Mg concentrations (0.74, 0.39, and 1.12 mM respectively) in these streams are higher than those measured in volcanic rock dominated watersheds in Panama and in first and second order streams flowing through basalts in Costa Rica. These differences reflect the lithologies and the ages of the rocks. N:P molar ratios vary from 6.4 to <1 in the Nicaraguan stream waters, suggesting these streams may be N-limited. In general, the highest phosphate concentrations were associated with lower Cl − values, implying that at least a portion of the P is derived from chemical weathering sources, although anthropogenic sources cannot be ruled out. The information presented herein represents some of the first data on small mountainous river systems in Nicaragua. The data provide information that may prove important for the understanding of the pre-Nicaragua canal conditions. Highlights: Some of the first stream geochemistry for rivers of western Nicaragua reported. High P concentrations and low N:P ratios in these rivers suggest P limitation. Highest dissolved silicate found in rivers draining slopes of active volcanism. Volcanic gases may be mediating weathering reactions in some rivers.