Arsenic Concentrations in Water Resources of the Choke Canyon/Lake Corpus Christi Reservoir System: Surface and Ground Waters
Jill Brandenberger, Patrick Louchouarn
During the last several decades, human-related activities and populations have increased markedly along the Texas Gulf Coast, intensifying pressures on the water resources and ecosystems of this area. The Lake Corpus Christi/Choke Canyon reservoir system mitigates issues of water quantity while perturbing the quality of available fresh water resources. A driving force in the degradation of water quality is the potential loading and deposition of colloidal particles with a high affinity for trace metals and their incorporation into bedded sediments of the system. This “sequestration” may not be permanent, as redox sensitive metals (Mn, Fe, U, As and Mo) are susceptible to diagenetic remobilization due to oscillations in the hydrodynamic regime. This process may in turn result in a positive flux of metals to the water column adding a substantial stress to the aquatic environment and decreasing the quality of our freshwater supply.
This research thus assessed the water quality of Lake Corpus Christi as a function of 1) tributaries and ground waters within the Nueces River basin, 2) present geochemical cycling of trace metals during temporal, spatial, and event driven oscillations of dissolved oxygen, and 3) sedimentary reservoirs in diverse sections of the lake (historic water quality as inferred from sediment cores).
Water column profiles for trace metals assessed seasonal variations (summer vs. winter), an inflow event (episodic floods) and spatial distributions (oxygenated vs. stratified water column). The hypolimnetic cycling of Mn, Fe, Pb, Cr, V, Co and Ni resulted in higher enrichment factors in summer vs. winter and at the deeper station (stratified water column). The strong correlation of Pb and Mn cycling suggests diagenetic remobilization of Pb. However, the mildly reducing conditions did not entrain the cycling of As, Mo and U. Moreover, whereas Mo and U concentrations remain constant at background levels within surface waters of the whole system of study, dissolved As values in the Lower Nueces River basin (8-12 µg/L), are enriched by two orders of magnitude with respect to background levels measured in the upper basin (0.5 µg/L). The conservative behavior of As results in seasonally cycling with dilution during periods of higher inflow (winter and spring) and evapoconcentration in the summer. This contributes to the degradation of water quality and results in seasonal concentrations above the recently adopted standard for arsenic in drinking water (10 µg/L).
Sediment cores encompassed the three reservoir zones: riverine, transition, and lacustrine, each with unique depositional environments. Average metal concentrations in surface sediments are below the threshold effect level (TEL) with the exception of Ni, but discrete depths indicate layers of enriched metal content. Normalized sediment profiles give further evidence of periods of enrichment and depletion of As, Co, Cu, Pb, Ni and Mn suggesting historic fluctuations in metal accumulation throughout the sediment profiles. Historical changes in material inputs to the sedimentary environments of the lake are further supported by significant changes in total organic matter and its isotopic signatures (d13C and d15N). Temporal trends, provided by the pre-reservoir conditions show decreasing values of Hg and Pb due to changes in atmospheric cycling and basinwide collection/redistribution processes. The uniform distribution of U and Mo around average sediment values suggests that the transport of these metals from upstream uranium mines has not impacted the sediments of Lake Corpus Christi. However, sedimentary peaks in As may support the selective transport and sequestration of this heavy metal due to uranium mining activities in Live Oak County.
The extreme heterogeneity of As, Mo and U in ground waters suggests a mechanism for sequestration, such as sulfidization upon contact with water discharging along faults in Live Oak County enriched in H2S. The concentrations of these metals in selected ground waters are elevated well above the drinking water standards. An emphasis was placed on As because elevated levels are seen in the surface waters as well as ground waters (~10 and ~20 µg/L, respectively) . However, the ground water samples provide the first indication of enrichment in U (47 µg/L) in the Nueces River basin well above drinking water standard (30 µg/L). Sources for these selective enrichments can include both anthropogenic activities such as past mining processes and agricultural pesticide used in the drainage/aquifer basin, and natural geological inputs to ground water reservoirs.