PROTECT: Puerto Rico Testsite for Exploring Contamination Threats
Modeling NAPL Transport in the North Coast Karst Aquifer, Puerto Rico
C. Butscher, R. Ghasemizadeh, I. Padilla, A. Alshawabkeh
Challenges in karst. Karst systems have a high degree of heterogeneity
(background picture and small photo), which makes them behave very
differently from other aquifers. Slow seepage through the rock matrix and
fast flow through conduits (Fig. 1) result in a high variation in aquifer
behavior. Contaminant storage occurs in the rock matrix, but contaminant
transport occurs mostly along preferential pathways in the conduit network
(Ghasemizadeh et al. in prep.). Computer models predicting contaminant
transport must be able to simulate combined matrix and conduit flow (Fig. 2).
The properties of the conduit network, however, are difficult to determine.
Why modeling? The prediction of water
resources and the assessment of
contaminant pathways toward exposure
demand quantitative simulation tools.
Groundwater models can be a major help in
determining potential paths for exposure of
superfund related contaminants in the North
Coast Limestone aquifer of Puerto Rico.
With this knowledge, strategies can be
undertaken to reduce exposure and protect
public health.
Dynamic transport of NAPLs. Nonaqueous phase liquids (NAPLs) are
important superfund-related contaminants.
They behave differently than water in the
subsurface: depending on their density, they
float and accumulate on the water table, or
tend to sink vertically to the aquifer basis.
The effect of different flow regimes on NAPL
transport is poorly understood in karst
systems (White 2002). Pooled NAPLs
remain stationary under base flow
conditions. During storm flows, NAPL pools
can be dragged downstream or flushed as
suspensions (Vesper et al. 2001). As a
result, storm flow can send previously
immobile NAPLs to outflows in toxic pulses.
A need exits to advance knowledge on the
main processes controlling NAPL fate and
transport in karst groundwater systems
characterized by variable conduit and
diffuse flow.
Fig. 1: Conceptual model of a karst
aquifer (Butscher and Huggenberger,
2009).
Fig. 2: “Bath tub” model symbolizing mixed reactor models to represent karst groundwater
systems (from Butscher and Huggenberger, 2009).
Fig. 4: Distributed finite difference model from the
Manatí – Vega Baja area, Puerto Rico (Cherry, 2001)
Suggested approach:
Distributed regional scale model of central part of North Coast Aquifer, PR (Fig.
3). Covers large area. Data collected by Project 4 and documented in
scientific reports (Cherry 2001).
Models at a smaller scale in Vega Alta region (Fig. 3). Extensive monitoring data
available.
Spatially lumped model (“mixed reactors”, Fig. 2). Applicable where conduit
system cannot be spatially resolved (Butscher and Huggenberger 2009).
Simulation of water and NAPL phase in one model.
Spatially distributed model (c.f., Fig.4). Finite difference model MODFLOW2005-CFP (Hill et al. 2010). Couples continuum (rock matrix) with discrete
conduit network to simulate combined conduit and diffuse flow. Interpretive
character because of unknown geometry of conduit network. Links
contamination with spatially distributed flow conditions. Combination with
lumped model allows minimizing interpretive character.
References:
Butscher C, Huggenberger P, 2009. Modeling the Temporal Variability of
Karst Groundwater Vulnerability, with Implications for Climate Change.
Environ. Sci. Technol. 43, 1665-1669.
Cherry GS, 2001. Simulation of flow in the upper north coast limestone
aquifer, Manatí-Vega Baja area, Puerto Rico. USGS Water-Resources
Investigations Report 00-4266, 82 p.
Ghasemizadeh R, Hellweger F, Butscher C, Padilla I, Vesper D, Field M,
Alshawabkeh A, in prep. Review: Groundwater flow and transport
modeling of karst aquifers, with particular reference to the North Coast
Aquifer of Puerto Rico. To be submitted to Hydrogeol. J.
Hill, ME, Stewart, MT, Martin, A, 2010. Evaluation of the MODFLOW-2005
Conduit Flow Process. Ground Water 48 (4), 549-559.
Vesper, DJ, Loop, CM, White, WB, 2001. Contaminant Transport in Karst
Aquifers. Theoretical and Applied Karstology 13-14, 101-111.
White, WB, 2002. Karst hydrology: Recent developments and open
questions. Eng. Geol. 65 (2–3), 85–105.
NAPL
pool
This program is supported by Award
Number P42ES017198 from the
National Institute of Environmental
Health Sciences.
Fig. 3: Suggested model areas in the North Coast Aquifer of Puerto Rico (map from Cherry 2001).
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