Karst systems have a high degree of heterogeneity and anisotropy, which makes them behave very differently from other aquifers. Slow seepage through the rock matrix and fast flow through conduits and fractures result in a high variation in spring response to precipitation events. Contaminant storage occurs in the rock matrix and epikarst, but contaminant transport occurs mostly along preferential pathways that are typically inaccessible locations, which makes modeling of karst systems challenging. Computer models for understanding and predicting hydraulics and contaminant transport in aquifers make assumptions about the distribution and hydraulic properties of geologic features that may not always apply to karst aquifers. This paper reviews the basic concepts, mathematical descriptions, and modeling approaches for karst systems. The North Coast Limestone aquifer system of Puerto Rico (USA) is introduced as a case study to illustrate and discuss the application of groundwater models in karst aquifer systems to evaluate aquifer contamination.

Abstract. More than 30 % of Europe's land surface is made up of karst exposures. In some countries, water from karst aquifers constitutes almost half of the drinking water supply. Hydrological simulation models can predict the largescale impact of future environmental change on hydrological variables. However, the information needed to obtain model parameters is not available everywhere and regionalisation methods have to be applied. The responsive behaviour of hydrological systems can be quantified by individual metrics, so-called system signatures. This study explores their value for distinguishing the dominant processes and properties of five different karst systems in Europe and the Middle East. By defining ten system signatures derived from hydrodynamic and hydrochemical observations, a processbased karst model is applied to the five karst systems. In a stepwise model evaluation strategy, optimum parameters and their sensitivity are identified using automatic calibration and global variance-based sensitivity analysis. System signatures and sensitive parameters serve as proxies for dominant processes, and optimised parameters are used to determine system properties. By sensitivity analysis, the set of system signatures was able to distinguish the karst systems from one another by providing separate information about dominant soil, epikarst, and fast and slow groundwater flow processes. Comparing sensitive parameters to the system signatures revealed that annual discharge can serve as a proxy for the recharge area, that the slopes of the high flow parts of the flow duration curves correlate with the fast flow storage constant, and that the dampening of the isotopic signal of the rain as well as the medium flow parts of the flow duration curves have a non-linear relation to the distribution of groundwater storage constants that represent the variability of groundwater flow dynamics. Our approach enabled us to identify dominant processes of the different systems and provided directions for future large-scale simulation of karst areas to predict the impact of future change on karst water resources.

[1] The main objective of this study was to quantify the intrinsic vulnerability of karst springs by numerical modeling. A global approach is used, modeling the discharge of a karst spring. This approach includes the hydrological dynamics of karst systems and is applicable to complex karst settings, where structural and hydraulic characteristics cannot be spatially resolved with sufficient accuracy. A basis model and four extended versions were set up to determine the individual characteristics of the present karst system and to include different flow processes that could affect the vulnerability of the system. All these model setups consider, besides recharge (soil and epikarst system), the conduit and the diffuse flow system as the main characteristics of the karst aquifer. The extended setups additionally account for surface runoff, an intermediate flow system, exchange flow between the conduit and the diffuse system and seasonal variation in the water storage capacity of the recharge system. Potential use of the calibrated models to quantify the intrinsic vulnerability of karst springs is discussed on the basis of (1) the temporally changing contributions of the conduit and diffuse flow systems to spring discharge, and (2) modeled breakthrough curves resulting from a standardized contaminant input into the karst system. The modeling approach complements vulnerability mapping methods by addressing temporal and quantitative aspects of vulnerability.Citation: Butscher, C., and P. Huggenberger (2008), Intrinsic vulnerability assessment in karst areas: A numerical modeling approach, Water Resour. Res., 44, W03408,

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