Estimating Preferential Flow in Karstic Aquifers Using Statistical Mixed Models

Abstract

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 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 statistical mixed models used in the study.

Characteristics of preferential flow paths and their effects on soil properties

“…In the subsoil, preferential flow domains act as conduits for the transport of Cu and Zn into the soil matrix but retain heterogeneously by adsorption on amorphous iron oxides that may dissolve and precipitate within macropores (Helmhart et al 2012). In this study, the lower soil metal content in the stained areas was caused by preferential flow, which can transport metal quickly (Subramanian and Cathles 2013) and reduce the adsorption of metal to soil particles (Anaya et al 2013). This confirms the results of Cheng et al (2009) in the Three Gorges area.…”

“…The effect of preferential flow on the spatial distribution of arsenic, copper and zinc was studied within the surface riverbed layer of a small stream that collects surface runoff from an arsenic-bearing waste pile of an abandoned mine, showing that in the subsoil, preferential flow paths were associated with lower copper and zinc concentrations and higher concentrations of arsenic (Helmhart et al 2012). Anaya et al (2013) found that preferential flow paths can increase soil organic matter content.…”

Effects of Grass and Forests and the Infiltration Amount on Preferential Flow in Karst Regions of China

“…The rapid process of preferential flow reduces the water retention time [18]; in order to make a profound study about the law of water cycle and the practical significance of natural forest and artificial forest on bottom of slope, the geochemical processes and the mode of vertical leakage need to be investigated [19]. At present, most studies that focus on different water inputs have lots of effects on preferential flow under different soil architectures [1], land cover [20], and land use [21].…”

Spatiotemporal changes of CVOC concentrations in karst aquifers: Analysis of three decades of data from Puerto Rico

“…The occurrence of CVOCs in groundwater systems poses a serious environmental threat to both natural ecosystem integrity and human water uses (Lapworth et al, 2012; Anaya et al, 2013). Exposure to CVOCs is potentially harmful to human health and may contribute to adverse reproductive outcomes (Sonenfeld et al, 2001; Forand et al, 2012), as well as damage to the nervous system (Bale et al 2011), liver and kidney (Lash et al, 2000), immune system (Cooper et al, 2009) and lungs (Odum et al, 1992; Chiu et al, 2013).…”