Modeling the Temporal Variability of Karst Groundwater

Precipitation and other meteorological data were recorded at an automatic climate station (operated by the University of Basel) and a daily sampled ra...
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Environ. Sci. Technol. 2009, 43, 1665–1669

Modeling the Temporal Variability of Karst Groundwater Vulnerability, with Implications for Climate Change CHRISTOPH BUTSCHER* PETER HUGGENBERGER University of Basel, Switzerland

SHUTTERSTOCK

To understand how groundwater resources may be affected by climate change and create sustainable management plans, modeling karst systems provides much insight.

Water is one of the most valuable natural resources of all, crucial to human and all other kinds of life. Changes in water quantity and quality are considered to have strong environmental and socio-economic consequences. Global warming due to the so-called “greenhouse effect” is reported to cause changes in precipitation patterns and evapotranspiration. In Europe, for example, it has been 10.1021/es801613g

 2009 American Chemical Society

Published on Web 03/12/2009

predicted that record-breaking heat waves, such as the one experienced in 2003, will become more frequent (1). At the same time, even as summers become drier, the incidence of severe precipitation events could increase (2). Freshwater resources are among the ecological systems that are particularly vulnerable to climate change (3). That many ecological systems rely on groundwater resources, only changes in hydrological regimes and water quality will have a strong environmental impact. For example, groundwater is a natural habitat for very specialized species, such as sightless and non-pigmented crawfish and cavefish, and it performs an important ecological function as a feeder of springs, rivers, and lakes. Additionally many public sectors and services such as water supply, agriculture, and industry depend on the availability of groundwater resources: shortfalls will have negative socioeconomic consequences. Climate-based hydrological changes can amplify the human-induced pressure exerted on future groundwater resources, including a growing population, changes in land use (urbanization, deforestation), or increasing industrial water demand. Hydrological research started to suggest in the late 1980s that changes in climatic variables might alter the availability of water resources and affect water quality (4). Since then, many studies have investigated the impact of climate change on groundwater resources, considering both temporal and spatial aspects. However, the focus of most of these studies was on the amount of water available, calculating recharge rates, and modeling groundwater levels on the basis of these rates (an overview is provided by ref (5)). These studies showed that the availability of groundwater will decrease in some regions and increase in others. However, the availability of groundwater required to meet demands is not a question of quantity alone. A very important factor for environmental consideration is also the groundwater quality. Water used for drinking supplies must fulfill certain standards. The availability of water resources must therefore imply the availability of adequate water in sufficient quality. Only a few of the studies linking climate change with groundwater resources have investigated the potential effect of climate change on water quality thus far. An important factor to be considered in conjunction with groundwater quality is the vulnerability of the resource, which is defined as the sensitivity of a groundwater system to pollution. Since the introduction of the concept of groundwater vulnerability (6), many strategies have been developed for vulnerability assessment and groundwater resource management. The proposed methods have also been integrated into environmental policy and regulation practices (7, 8), with intrinsic vulnerability being the emphasis. Intrinsic vulnerability refers to the sensitivity to pollution when considering only natural, geogenic conditions without the effects of human activities such as contaminant release.

Modeling Vulnerability Traditionally, the strategies to assess intrinsic vulnerability have been based on a “static” concept: that vulnerability is VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Conceptual model of a karst aquifer. a characteristic of groundwater systems that does not change over time. Such a concept allows for vulnerability to be represented by areas of similar attributes on a geographic map. However, intrinsic vulnerability strongly depends on the recharge conditions, which are not only a matter of the surface and subsurface structure of the ground, but also dependent on precipitation and evaporation patterns. Groundwater recharge is highly time dependent, and the future evolution of groundwater vulnerability in a changing climate can be evaluated only if vulnerability concepts are “dynamic”. That is, groundwater variations have both spatial and temporal components. The application of a dynamic concept of vulnerability to groundwater modeling requires a new methodology. For example, tracking the movement of water through aquifers and discharge at springs becomes a task of considering interacting flow systems. Processes of interest include the temporal variation of groundwater recharge, its distribution to various flow systems, and storage in these systems with different residence times. Understanding these processes is necessary to the management of groundwater resources. It is therefore crucial to better quantify groundwater vulnerability via numerical modeling to achieve a broader understanding of how climate change may affect this vital resource.

Karst Groundwater Vulnerability Karst groundwater is one of the most important freshwater resources. Waters from karst aquifers supply about 25% of the global population with water (9). In Europe, a significant portion of the drinking water supply is extracted from karst aquifers, and in many regions it is the only available source of freshwater (10). Although the ecological function of karst groundwater is at an early stage of being recognized, the awareness that karst aquifers are particularly vulnerable to contamination is rapidly taking hold (10). Furthermore, karst springs are very sensitive to changes in the hydrology of the recharge areas. The porosity of the rocky landscapes comprising a karst means that the interrelationship between surface and subsurface is more pronounced. Therefore, it is expected that the effects of climate change on groundwater vulnerability can be observed in particular at karst springs. Karst aquifers exhibit a “dual” conduit and diffuse flow system (11): water flows in pipe-like conduits as well as through fractures and pores (Figure 1). This dual characteristic of karst systems causes the vulnerability of karst systems to be dual as well. On the one hand, karst aquifers are especially sensitive to short-lived contaminants because of the fast travel times and low storage capacity in the conduit 1666

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system, causing natural mitigation processes like adsorption, degradation, and filtration to have little effect. On the other hand, persistent pollutants can be stored and released from the diffuse system over a long period (12). The sensitivity of such systems to short-lived contaminants is therefore fundamentally different from their sensitivity to persistent contaminants. Thus vulnerability to contaminants is a binary concept for karsts: conduit flow vulnerability (short-term contamination) is distinct from diffuse flow vulnerability (persistent contamination), yet both effects are manifest at the point of discharge. Thus, this dual approach to karst groundwater vulnerability does not contradict the concept of intrinsic (karst) groundwater vulnerability that historically has not taken the properties of a contaminant into account. Typically, microbes perish in the karst flow system within a few days. Under certain conditionssfor example, if an adequate substrate such as cave stream sediment is availablesmicrobes can be viable in the conduit flow system for up to many months. However, even then the occurrence of microbes in spring water is strongly related to fast flow conduits. Microbes are therefore related to the conduit flow vulnerability. In this context, fecal bacteria, which are a common problem for the drinking water quality of many karst spring captures, can serve as a typical example of shortlived contaminants. By contrast, many pesticides only slowly degrade and are a typical example of persistent contaminants. The numerical modeling approach chosen here has recently been proposed by the authors (13). It considers karst groundwater vulnerability to be a dynamic response to variable precipitation and evaporation conditions. The methodology is designed to quantify the temporal variation of vulnerability and represents an appropriate tool to assess the impact of climate change on karst groundwater vulnerability. In this study, we present an example of this approach from a field site in Switzerland to predict the potential impacts of climate change on the vulnerability of the karst groundwater resources at this site. We discuss our findings in light of the challenges and chances presented by present-day groundwater management, and adapt the conclusions reached in our example to the future state of sustainable water resources.

Example from Switzerland Study Area. The spring selected for this study is located 10 km southeast of the city of Basel in northwest Switzerland (Untere Rappenfluhquelle). It is one of several spring captures which together drain a catchment of approximately 1 km2. The catchment is situated on a karst plateau of the Swiss Tabular Jura, a low mountain range composed of predominantly flat-lying Triassic and Jurassic sediments (Gempen Plateau). The aquifer comprises massive Oxfordian limestone of varying thickness (40-70 m) with high primary porosity, representing a patch-reef facies of a shallow carbonate platform. The aquifer base is made up of low permeability Oxfordian marls approximately 100 m thick. The aquifer is fractured and highly karstic. The hydrogeology of the study area is characterized by karst water circulation in an unconfined setting. According to the hydrogeologic evolutionary typology for karsts (14), the setting corresponds to an exposed, open karst. Numerical Model. The recharge, conduit flow, diffuse flow (RCD) rainfall-discharge model “RCD-seasonal” (13) was used to simulate the discharge and substance concentration of the investigated spring. This lumped parameter model is based on a conceptual model of karst groundwater systems in which flow can be attributed to three distinct compartments, representing the characteristic flow systems involved: the recharge system (soil and epikarst system), the conduit (fast) flow system, and the diffuse (slow) flow system (Figure 1). These characteristic variable volume compartments are

FIGURE 2. Schematic representation of the model setup. the inspiration for the acronym RCD and are connected by links conceptually depicted in Figure 2 and described below. The used “conduit and diffuse” concept of karst groundwater flow (11) is widely accepted and confirmed, but not universally embraced by karst researchers. For this reason, other model setups were additionally tested, and the model with the best fit of simulated to measured spring discharge was chosen for this study. Input into the recharge system was estimated by measured precipitation. The water in the soil and epikarst system is available for evapotranspiration. Groundwater recharge to the underlying karst systems (conduit and diffuse, Figure 2) occurs when the water content of the recharge system exceeds the retention capacity of this storage. In our example, the retention capacity was found to be seasonally variable. A smaller portion of the groundwater recharge flows into the conduit system, while the rest enters the diffuse system. The conduit and diffuse systems both contribute to the spring discharge proportional to their water volume (linear outflow), with a higher outflow coefficient (shorter residence time) in the conduit system. The model is calibrated by applying a numerical nonlinear regression algorithm to minimize deviations between measured and calculated spring discharge (13). Model simulation and parameter estimation was performed using the AQUASIM software (version 2.1e) (15). Details about the evaluation of the model setup, model formulation, and the equations used by the model to calculate flow and substance loads are summarized in the Supporting Information. Quantification of Vulnerability. The calibrated model is used to quantify the intrinsic vulnerability of a karst spring as well as its variation in time. The quantification is based on (1) the vulnerability index VI, representing varying relative contributions of the conduit and diffuse flow system to spring discharge, and (2) the modeled vulnerability concentration CV in spring water resulting from a standardized contaminant input into the system. VI is calculated to quantify the “conduit flow vulnerability” to short-lived contaminants (e.g., fecal bacteria), whereas CV is calculated to quantify the “diffuse

flow vulnerability” to persistent contaminants such as pesticides (13, 16). Karst aquifers are especially vulnerable because of a close interrelation between surface water and groundwater reflected in fast travel times and low storage capacity in the conduit system. Processes such as adsorption and degradation are relatively ineffective in this fast flow system unlike in the diffuse flow system. Therefore vulnerability may be reduced by dilution of conduit system water by the diffuse flow system. Thus the vulnerability index VI is defined as the ratio of conduit to diffuse contributions as a quantitative indicator of karst groundwater vulnerability and its variation in time. The occurrence of high VI values is therefore a sign of high conduit flow vulnerability: the spring is highly sensitive to contamination by short-lived contaminants at a given time. Because the application of VI is restricted to short-lived contaminants, the vulnerability concentration CV is additionally defined as the simulated concentration of a nondegradable contaminant in spring water resultant of a continuous contaminant input. CV represents the impact of a contaminant that is not effectively mitigated by adsorption or degradation in the diffuse system (e.g., pesticides). Just as in the case of VI, the level of CV is a quantitative indicator of karst groundwater vulnerability at the spring and its variation in time. The occurrence of high levels of CV is a sign of high diffuse flow vulnerability: indicating that the spring is highly sensitive to contamination by persistent contaminants at a given time. To analyze the results and for visualization purposes, VI and CV can be plotted against time. Details on how VI and CV are calculated by our model are in the Supporting Information; a thorough example of how VI and CV can be applied to a karst spring catchment to result in an enhanced vulnerability assessment is given in ref (16). Data and Simulated Scenarios. Water levels at a V-notch weir were recorded continuously with a pressure transducer. Discharge was calculated using the relation Q ) kh2.5

(1)

where Q is the discharge, k is a constant depending on the weir’s geometry, and h is the water level (17). Precipitation and other meteorological data were recorded at an automatic climate station (operated by the University of Basel) and a daily sampled rain gauge (operated by MeteoSwiss). For months with snowfall, a rainfall equivalent that considered solid precipitation and runoff generated by snowmelt was calculated based on the temperature index method (18). Calculation of the potential evapotranspiration was based on the Penman equation (19) and incorporated air temperature, shortwave downward radiation, wind velocity, and relative humidity. Simulations were conducted using a daily step time series. Present vulnerability was estimated using field data from a period of 461 days from May 28, 2004 to August 31, 2005 (investigation period). To minimize the influence of initial storage volumes on the model results, the simulation was started 240 days before the investigation period on October 1, 2003 as a “warming-up” period. Meteorological data, but no discharge data, were available for the warming-up period. Two climate change scenarios were simulated and compared to the results obtained with the present climatic conditions observed during the simulation period. The first scenario represents hot and dry summers, which are predicted to become more frequent in Europe as a result of the greenhouse effect (1). Precipitation and evapotranspiration data in the city of Basel from January to August 2003 (provided by MeteoSwiss) were used as model input. The summer of 2003 was characterized by a heat wave in central Europe and can be used in climate impact studies as an analog of summers predicted for coming decades (20). VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Simulated vulnerability index (VI) illustrating the expected effects of predicted climate change on “conduit flow vulnerability”. Conduit flow vulnerability represents the sensitivity of the groundwater system to short-lived pollutants (e.g., fecal bacteria). The second scenario represents severe rainfall events. We implemented two days of heavy rainfall at the beginning of the simulation period by increasing the measured daily rainfall on the second and third of June 2004 from 10.4 and 29.7 mm, respectively, to 100.0 mm each for these days. This scenario is comparable to the extreme weather situation in central Switzerland in August 2005, when all climate stations recorded more than 100 mm rainfall, with most at even more than 200 mm and some up to 300 mm in 48 h. This event caused heavy damages from flooding and landslides, and financial losses of approximately $3.0 billion (USD) (21). The frequency of such extreme weather situations in Europe is considered to increase (22). Effects of Predicted Climate Change on Karst Groundwater Vulnerability. The anticipated effects of predicted climate change on the conduit flow vulnerability of the study area, represented by the vulnerability index VI, are illustrated in Figure 3. The simulation using the scenario “summer heat wave” results in generally shorter and less frequent periods with a high VI as opposed to the simulation that was conducted with the observed (present-day) climate conditions from the simulation period. The effect is most obvious during spring and summer. The simulations indicate that the impact of short-lived contamination (e.g., fecal bacteria) is likely to decrease in the study area with the projected increased occurrence of hot and dry summers in the future. The results from the scenario “severe rainfall event” are in some respects astonishing. As expected, the conduit flow vulnerability rises dramatically directly after the event. In the long term however, the conduit flow vulnerability slightly decreases as a result of enhanced recharge (also) to the diffuse flow system during the event. This recharge water is stored over quite some time, providing for an enhanced dilution of water from the conduit system during later events. Overall, this effect is advantageous for the drinking water supply: the long-term water quality is improved, while the short-term endangering of the water quality can be managed in most cases (e.g., by adapted raw water treatment such as chlorination or ozonation or by transient raw water rejection). Figure 4 summarizes the expected effects of predicted climate change on the diffuse flow vulnerability of the study area, represented by the vulnerability concentration CV. The simulation using the scenario “summer heat wave” results in higher CV levels compared to the simulation conducted with the observed (present-day) climate conditions. The simulations indicate that the impact of persistent contamination (e.g., pesticides) is likely to increase in the study area with an increased occurrence of hot and dry summers in the future. However, the effect of severe rainfall events is the opposite. As noted above, dilution effects due to the enhanced groundwater recharge during the extreme event provide for 1668

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FIGURE 4. Simulated vulnerability concentration (CV) illustrating the expected effects of predicted climate change on “diffuse flow vulnerability”. Diffuse flow vulnerability represents the sensitivity of the groundwater system to persistent pollutants (e.g., pesticides). a lower impact of persistent contaminants in the study area when factoring in the expected increase in the occurrence of extreme rainfall events in the future. These surprising results show that climate change may or may not be deleterious to groundwater vulnerability. However, optimism must be tempered because which of the two countering processes becomes most relevant to groundwater vulnerability will depend on the future frequency and intensity of the anticipated scenarios. Computations show that the reliable forecasting of future regional climatic conditions is a crucial prerequisite for predicting the future evolution of groundwater vulnerability at a given site.

Challenges and Chances The presented example of modeling future groundwater vulnerability illustrates the present-day challenges and chances to manage evolving water resources in a changing climate. Because groundwater vulnerability depends on local recharge conditions and subsurface properties, the modeled impact of climate change on groundwater vulnerability is likely to be site specific. The results we obtained from the test site indicate a decrease in short-lived contaminants in spring water as a result of climate change. The impact of persistent contaminants, however, can only be determined if future climatic conditions at the site can be estimated with sufficient accuracy, because predicted summer heat waves and severe rainfall events will have opposite effects on the groundwater vulnerability. Nevertheless, there are also general conclusions that can be drawn from this study. The presently large uncertainties that still exist in regional climate models (23) interfere with uncertainties in the groundwater models. A possible way to reduce model uncertainties is to improve the necessary data base by the establishment of more monitoring systems. Accurate evaluation of regional precipitation and evapotranspiration data will then provide models with more input data and decrease uncertainties as to vulnerability assessment. The presented time-variant modeling approach itself is well-suited for the spatial and temporal scales that are relevant for water suppliers both for short and long terms. It can be adapted to local factors that will impact future groundwater quality, such as nutrient leaching in agricultural zones or mined areas. Data requirements are moderate and model handling is straightforward for professionals. Sitespecific knowledge of the evolving groundwater vulnerability in a changing climate will enable water suppliers and local authorities to take the necessary steps toward a sustainable

and proactive management of the water resources. These steps could include measures to preserve drinking water quality by tightened land use restrictions in the recharge areas, or the installation of adequate water treatment systems. As climate change impacts on hydrological processes have been observed already (24), the appropriate time to consider them is now. Christoph Butscher is a research scientist at the department of environmental sciences, Applied and Environmental Geology Group, University of Basel, Switzerland. Peter Huggenberger is professor of applied geology at the University of Basel, Switzerland, head of the Applied and Environmental Geology Group and in charge of the geological survey of Basel, Switzerland. Please address correspondence on this article to [email protected].

Acknowledgments We thank the Institute of Meteorology, Climate and Remote Sensing (University of Basel) and MeteoSwiss for the provision of meteorological data. The thorough and constructive comments made by the three reviewers are highly acknowledged.

Supporting Information Available More detailed information on the modeling approach. This material is available free of charge via the Internet at http:// pubs.acs.org.

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