PROTECT: Puerto Rico Testsite for Exploring Contamination
Threats www.neu.edu/protect 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. Modeling NAPL
Transport in the North Coast Karst Aquifer, Puerto Rico C.
Butscher, R. Ghasemizadeh, I. Padilla, A. Alshawabkeh This program
is supported by Award Number P42ES017198 from the National
Institute of Environmental Health Sciences. 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).
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. 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, Manat-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 (23), 85105. Dynamic transport of NAPLs.
Non- aqueous 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.
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 MODFLOW- 2005-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. Fig. 3: Suggested model areas in the North Coast Aquifer
of Puerto Rico (map from Cherry 2001). Fig. 4: Distributed finite
difference model from the Manat Vega Baja area, Puerto Rico
(Cherry, 2001) NAPL pool
