Comparison of microbial indicators and seasonal temperatures as means for evaluating the vulnerability of water resources from karst and siliciclastic springs

Hydrogeology Journal - Spring water is a critical resource in many parts of the world, however, there are few effective and efficient means of evaluating the vulnerability and sustainability of...     

Geochemistry of a spring-dense karst watershed located in a complex structural setting, Appalachian Great Valley, West Virginia, USA

The distribution and chemistry of the springs in the Tuscarora Creek watershed is controlled by both geologic structure and karst dissolution. The watershed is located in eastern West Virginia in the structurally complex Great Valley of the Appalachian Valley and Ridge province. The upper portion of the stream parallels strike along a mapped fault zone and is bordered by clastic rocks that comprise North Mountain. The lower reaches of the stream flow cross-strike through Cambro-Ordovician carbonates. The controlling chemical signature in the spring water is carbonate dissolution. Little evidence was seen for the recharge from adjacent clastic rocks although differences in the Ca/Mg molar ratio between springs indicated the presence of localized spring basins in headwater reaches. Na, Cl and Ca generally increased from upstream to downstream in the cross-strike reaches. Comparison of stream and cumulative spring discharge was consistent with significant groundwater base-flow contribution directly to the creek, particularly in the strike-parallel region. The largest spring in the watershed (>162 L/s) was sampled during and after a large storm event along with the adjacent creek. The creek displayed a typical dilution response with each flood pulse, whereas the spring had only a limited or delayed response. The overall chemical and thermal stability of the spring, relative to the creek, indicated the lack of significant direct hydraulic connection between the two. The conceptual model for the area includes localized flowpaths in the headwater region where the stream flow is parallel to strike and a thrust fault. In addition to the shallow localized flowpaths, a deeper, more regional flowpath likely exists for a large spring further downstream.     

Estimating Preferential Flow in Karstic Aquifers Using Statistical Mixed Models

Karst aquifers are highly productive groundwater systems often associated with conduit flow. These systems can be highly vulnerable to contamination, resulting in a high potential for contaminant exposure to humans and ecosystems. This work develops statistical models to spatially characterize flow and transport patterns in karstified limestone and determines the effect of aquifer flow rates on these patterns. A laboratory‐scale Geo‐HydroBed model is used to simulate flow and transport processes in a karstic limestone unit. The model consists of stainless steel tanks containing a karstified limestone block collected from a karst aquifer formation in northern Puerto Rico. Experimental work involves making a series of flow and tracer injections, while monitoring hydraulic and tracer response spatially and temporally. Statistical mixed models (SMMs) are applied to hydraulic data to determine likely pathways of preferential flow in the limestone units. The models indicate a highly heterogeneous system with dominant, flow‐dependent preferential flow regions. Results indicate that regions of preferential flow tend to expand at higher groundwater flow rates, suggesting a greater volume of the system being flushed by flowing water at higher rates. Spatial and temporal distribution of tracer concentrations indicates the presence of conduit‐like and diffuse flow transport in the system, supporting the notion of both combined transport mechanisms in the limestone unit. The temporal response of tracer concentrations at different locations in the model coincide with, and confirms the preferential flow distribution generated with the SMMs used in the study.