Mean Transit Time Of Water Research Articles

Long-Term Variations in Spring Water Mean Transit Time in a Forested Headwater Catchment in Japan

In this study, we applied a new methodology that utilized stable isotopic concentrations of water and SF6 concentrations of the air to investigate temporal variations in the spring water mean transit time (MTT) of a forested headwater catchment in Japan. We monitored the area from June 2010 to November 2021, including the forest thinning that occurred in October 2011. The SF6 concentrations were used to estimate the apparent age of the spring water, which was used to initiate a parameter calibration that determined the best possible MTT for each study period. The MTT was evaluated by estimating the d-excess variations of the spring water stable isotopes using the exponential piston flow model. The MTT ranged from 40 to 55 months and tended to increase soon after the forest was thinned. In contrast, the MTT was shorter by approximately 6 years after thinning occurred. The results indicate that the characteristics of the hydrological processes in the catchment underwent long-term changes after the forest was thinned. This study demonstrates that using combined tracer methods to investigate the hydrological response to forest management practices improved the results and can be used for better forest and subsurface water resource management.

Exploring a Climate Gradient of Midwestern USA Agricultural Landscape Runoff Using Deuterium (δD) and Oxygen-18 (δ18O)

Intensive agricultural activities have altered the hydrology of many Midwestern USA landscapes. Drain tiles (subsurface corrugated and perforated flexible tubing) has changed how and when water is discharged into the streams. Stable isotopes of oxygen (oxygen-18) and hydrogen (deuterium) were used to investigate hydrologic characteristics of three intensively managed agricultural landscapes in southern Minnesota (MN) and South Dakota (SD). Monthly δD and δ18O samples were collected across a climatic gradient from March 2016 to March 2018. Local meteoric water lines were established for the comparison of precipitation and evaporation magnitude from different sources at each location. These included vadose zone, phreatic zone, deep groundwater, tile drain, and river source waters. Two end-member hydrograph separation was performed at each site on selected dates to partition the shallow groundwater tile drainage contribution to streamflow. A lumped parameter modeling approach was applied to each dataset to investigate the mean transit time of water through different zones. Local meteoric water lines demonstrated differences in isotopic signatures due to the climate gradient to show the impact of low humidity and less rainfall. The hydrograph separation results showed that, from west South Dakota to eastern Minnesota, tile drains contributed about 49%, 64%, and 50% of the watershed streamflow. Precipitation took an average of 9 months to move through different pathways to end up in groundwater and an average of 4 months to end up in tile drains. This study confirms the important role tile drains play in the intensively managed fields and watersheds of Midwestern agriculture.

Sources and mean transit times of intermittent streamflow in semi-arid headwater catchments

Determining the sources and mean transit times (MTTs) of water that generates streamflows is important for understanding and managing headwater catchments. The sources and especially the MTTs of water that contributes to streamflow in seasonally intermittent streams are far less studied than for perennial streams. Here we use major ions, dissolved organic carbon, stable isotopes, radon and tritium to quantify the sources and MTTs of intermittent streamflows in three headwater catchments (Banool, McGill and Plantation) from a semi-arid area in southeast Australia. At the start of streamflows, the MTTs of stream water varied from 25 to 42 years in the 1.51 km2 Banool catchment, 3 to 4 years in the 3.38 km2 McGill catchment and 9 to 14 years in the 3.41 km2 Plantation catchment. Winter rainfall increased the relative contribution of younger waters in all three catchments. During higher winter streamflows, the MTTs of stream water reduced to ≤5 years in the Banool catchment and <1 year in the McGill and Plantation catchments. The sources of streamflow also differed between the catchments. Regional groundwater, which close to the stream has a residence time of several hundred years, dominated in the Banool catchment, whereas younger water (residence times of up to 9 years) stored in the riparian zone was the main source in the McGill and Plantation catchments. The differences in MTTs between the catchments may reflect land-use differences, especially the presence of plantation forests in the McGill and Plantation catchments. Overall, due to being less well-connected to the regional groundwater, the MTTs of these intermittent streams are far shorter than those reported for perennial headwater streams in southeast Australia. The short MTTs indicate that these intermittent streams are vulnerable to short-term variations in rainfall.

Water storage, mixing, and fluxes in tile-drained agricultural fields inferred from stable water isotopes

Quantifying hydrological processes that control the upper critical zone water balance and contaminant transport in drained landscapes is needed, especially as precipitation patterns driving water balance dynamics continue to shift due to climate change. Here, hydrometric data are integrated with stable isotope signatures to quantify water storage, mixing, and fluxes to subsurface tile drainage at an agricultural field located in Indiana, USA. Over a 2-yr period, precipitation, soil water sampled with suction lysimeters (10–80 cm depth), groundwater (below tile depth; >1 m), and subsurface tile discharge were sampled 97 times. Results showed that isotopic variability in near-surface soil water (10–20 cm) reflected the seasonality of the precipitation input signal, while groundwater values were relatively consistent indicating that water stored below tile drain depth was recharged during winter. Soil water between 20 and 80 cm depth was a mixture of near-surface water and groundwater that varied seasonally depending upon groundwater hydrodynamics. Mean transit time of water ranged from 12 to 20 d for 10-cm soil water to 225–334 d for groundwater, with tile drainage exhibiting a mean transit time of 245 d. Both two- and three-component hydrograph separation indicated that groundwater was the primary source of water to the tile drain followed by soil water. Tile drain hydrograph response (i.e., celerity) was largely controlled by antecedent wetness. Comparison of tile drain celerities and velocities revealed however varying mechanisms controlling hydrograph response across a range of environmental conditions. Data sets of both water and tracer flux were, thus, useful to track the spatiotemporal variability of water fluxes within and from the critical zone. Such data provide valuable information to improve the representation of critical zone processes in these landscapes within spatially distributed hydrological models.

Modifying the Jackson index to quantify the relationship between geology, landscape structure, and water transit time in steep wet headwaters

AbstractThe relationship between stream water mean transit time (MTT), catchment geology, and landscape structure is still poorly characterized. Here, we present a new simple index that builds on the Jackson, Bitew, and Du (2014) index that focuses specifically on permeability contrasts at the soil–bedrock interface and digital elevation model‐based physical flow path measurements to identify broad landscape trends of moisture redistribution in the subsurface of steep wet headwater catchments. We use this index to explore the relationship between geology, landscape structure, and water transit time through the lens of landscape anisotropy. We hypothesize that catchments with a greater tendency to shed water laterally will correlate with younger stream water MTT and catchments with a greater tendency to infiltrate water vertically will correlate with older stream water MTT. We tested the new index at eight geologically diverse Pacific Rim catchments in Oregon, Japan, and New Zealand. The new index explained 77% of the variability in measured stream water MTT across these varied sites. These findings suggest that critical zone anisotropy and catchment form are first‐order controls on the time scales over which catchments store and release their water and that a simple index may usefully capture this relationship.

Using isotopes to understand the evolution of water ages in disturbed mixed land‐use catchments

AbstractTracer studies have been key to unravelling catchment hydrological processes, yet most insights have been gained in environments with relatively low human impact. We investigated the spatial variability of stream isotopes and water ages to infer dominant flow paths in a ~10‐km2 nested catchment in a disturbed, predominantly agricultural environment in Scotland. We collected long‐term (&gt;5 years) stable isotope data of precipitation, artificial drainage, and four streams with varying soil and land use types in their catchment areas. Using a gamma model, Mean Transit Times (MTTs) were then estimated in order to understand the spatial variability of controls on water ages. Despite contrasting catchment characteristics, we found that MTTs in the streams were generally very similar and short (&lt;1 year). MTTs of water in artificial drains were even shorter, ranging between 1 to 10 months for a typical field drain and &lt;0.5 to 1 month for a country road drain. At the catchment scale, lack of heterogeneity in the response could be explained by the extensive artificial surface and subsurface drainage, “short‐circuiting” younger water to the streams during storms. Under such conditions, additional intense disturbance associated with highway construction during the study period had no major effect on the stream isotope dynamics. Supplementary short‐term (~14 months) sampling of mobile soil water in dominant soil‐land use units also revealed that agricultural practices (ploughing of poorly draining soils and soil compaction due to grazing on freely draining soils) resulted in subtle MTT variations in soil water in the upper profile. Overall, the isotope dynamics and inferred MTTs suggest that the evolution of stream water ages in such a complex human‐influenced environment are largely related to near‐surface soil processes and the dominant soil management practices. This has direct implications for understanding and managing flood risk and contaminant transport in such environments.

Characterizing Water Flow in Vegetated Lysimeters with Stable Water Isotopes and Modeling.

We have used stable water isotopes (δ18 O, δ2 H) in combination with lumped-parameter modeling for characterizing unsaturated flow in two lysimeters vegetated with maize. The lysimeters contained undisturbed soil cores dominated by sandy gravel (Ly1) and clayey sandy silt (Ly2). Stable water isotopes were analyzed in precipitation and lysimeter outflow water over about 3years. The mean transit time of water T and dispersion parameter PD , obtained from modeling, were higher for the silt soil in Ly2 than for the gravel soil in Ly1 (T of 362 vs. 129 d, PD of 0.7 vs. 0.12). The consideration of preferential flow (PF) paths could substantially improve the model curve fits, with 13 and 11% contribution of PF for Ly1 and Ly2 as best estimates. Different assumptions were compared to estimate the input function, that is, stable water isotope content in the recharging water. Using the isotopic composition of precipitation as input (no modification) resulted in reasonable model estimations. Best model fits for the entire observation were obtained by weighting the recharging isotopes according to average precipitation within periods of 3 and 6months, in correspondence to changing vegetation phases and seasonal influences. Input functions that consider actual evapotranspiration could significantly improve modeling at some periods, however, this led to deviations between modeled and observed δ18 O at other periods. This may indicate the influence of variable flow, so that dividing the whole observation period into hydraulically characteristic sub-periods for lumped-parameter modeling (which implements steady-state flow) is recommended for possible further improvement.

Assessing the controls and uncertainties on mean transit times in contrasting headwater catchments

Estimating the time required for water to travel through headwater catchments from where it recharges to where it discharges into streams (the transit time) is important for understanding catchment behaviour. This study uses tritium (3H) activities of stream water to estimate the mean transit times of water in the upper Latrobe and Yarra catchments, southeast Australia, at different flow conditions. The 3H activities of the stream water were between 1.26 and 1.99 TU, which are lower than those of local rainfall (2.6 to 3.0 TU). 3H activities in individual subcatchments are almost invariably lowest at low streamflows. Mean transit times calculated from the 3H activities using a range of lumped parameter models are between 7 and 62 years and are longest during low streamflows. Uncertainties in the estimated mean transit times result from uncertainties in the geometry of the flow systems, uncertainties in the 3H input, and macroscopic mixing. In addition, simulation of 3H activities in FEFLOW indicates that heterogeneous hydraulic conductivities increase the range of mean transit times corresponding to a specific 3H activity. The absolute uncertainties in the mean transit times may be up to ±30 years. However, differences between mean transit times at different streamflows in the same catchment or between different subcatchments in the same area are more reliably estimated. Despite the uncertainties, the conclusions that the mean transit times are years to decades and decrease with increasing streamflow are robust. The seasonal variation in major ion geochemistry and 3H activities indicate that the higher general streamflows in winter are sustained by water displaced from shallower younger stores (e.g., soils or regolith). Poor correlations between 3H activities and catchment area, drainage density, mean slope, distance to stream, and landuse, imply that mean transit times are controlled by a variety of factors including the hydraulic properties of the soils and aquifers that are difficult to characterise spatially. The long mean transit times imply that there are long-lived stores of water in these catchments that may sustain streamflow over drought periods. Additionally, there may be considerable delay in contaminants reaching the stream.

Contrasting transit times of water from peatlands and eucalypt forests in the Australian Alps determined by tritium: implications for vulnerability and the source of water in upland catchments

Abstract. Peatlands are a distinctive and important component of many upland regions that commonly contain distinctive flora and fauna which are different from those of adjacent forests and grasslands. Peatlands also represent a significant long-term store of organic carbon. While their environmental importance has long since been recognised, water transit times within peatlands are not well understood. This study uses tritium (3H) to estimate the mean transit times of water from peatlands and from adjacent gullies that contain eucalypt forests in the Victorian Alps (Australia). The 3H activities of the peatland water range from 2.7 to 3.3 tritium units (TUs), which overlap the measured (2.9 to 3.0 TU) and expected (2.8 to 3.2 TU) average 3H activities of rainfall in this region. Even accounting for seasonal recharge by rainfall with higher 3H activities, the mean transit times of the peatland waters are &lt; 6.5 years and may be less than 2 years. Water from adjacent eucalypt forest streams has 3H activities of 1.6 to 2.1 TU, implying much longer mean transit times of 5 to 29 years. Cation ∕ Cl and Si ∕ Cl ratios are higher in the eucalypt forest streams than the peatland waters and both of these water stores have higher cation ∕ Cl and Si ∕ Cl ratios than rainfall. The major ion geochemistry reflects the degree of silicate weathering in these catchments that is controlled by both transit times and aquifer lithology. The short transit times imply that, unlike the eucalypt forests, the peatlands do not represent a long-lived store of water for the local river systems. Additionally, the peatlands are susceptible to drying out during drought, which renders them vulnerable to damage by the periodic bushfires that occur in this region.

Using tritium to document the mean transit time and sources of water contributing to a chain-of-ponds river system: Implications for resource protection

Documenting the interaction between groundwater and rivers is fundamental to understanding hydrological systems. While many studies have examined the location and magnitude of groundwater inflows to rivers, much less is known about the transit times of water in catchments and from where in the aquifer the groundwater originates. Resolving those questions is vital for protecting riverine ecosystems, assessing the impact of contamination, and understanding the potential consequences of groundwater pumping. This study uses tritium (3H) to evaluate the mean transit times of water contributing to Deep Creek (southeast Australia), which is a chain-of-ponds river system. 3H activities of river water vary between 1.47 and 2.91 TU with lower 3H activities recorded during cease-to-flow periods when the river comprises isolated groundwater-fed pools. Regional groundwater 1–2.5 km away from Deep Creek at depths of 7.5–46.5 m has 3H activities of between <0.02 and 0.84 TU. The variation in 3H activities suggest that the water that inflows into Deep Creek is dominated by near-river shallow groundwater with the deeper groundwater only providing significant inflows during drier periods. If the water in the catchment can be represented by a single store with a continuum of ages, mean transit times of the river water range between <1 and 31 years whereas those of the groundwater are at least 75 years and mainly >100 years. Alternatively the variation in 3H activities can be explained by mixing of a young near-river water component with up to 50% older groundwater. The results of this study reinforce the need to protect shallow near-river groundwater from contamination in order to safeguard riverine ecosystems and also illustrate the potential pitfalls in using regional bores to characterise the geochemistry of near-river groundwater.

Insights into the water mean transit time in a high-elevation tropical ecosystem

Abstract. This study focuses on the investigation of the mean transit time (MTT) of water and its spatial variability in a tropical high-elevation ecosystem (wet Andean páramo). The study site is the Zhurucay River Ecohydrological Observatory (7.53 km2) located in southern Ecuador. A lumped parameter model considering five transit time distribution (TTD) functions was used to estimate MTTs under steady-state conditions (i.e., baseflow MTT). We used a unique data set of the δ18O isotopic composition of rainfall and streamflow water samples collected for 3 years (May 2011 to May 2014) in a nested monitoring system of streams. Linear regression between MTT and landscape (soil and vegetation cover, geology, and topography) and hydrometric (runoff coefficient and specific discharge rates) variables was used to explore controls on MTT variability, as well as mean electrical conductivity (MEC) as a possible proxy for MTT. Results revealed that the exponential TTD function best describes the hydrology of the site, indicating a relatively simple transition from rainfall water to the streams through the organic horizon of the wet páramo soils. MTT of the streams is relatively short (0.15–0.73 years, 53–264 days). Regression analysis revealed a negative correlation between the catchment's average slope and MTT (R2 = 0.78, p &lt; 0.05). MTT showed no significant correlation with hydrometric variables, whereas MEC increases with MTT (R2 = 0.89, p &lt; 0.001). Overall, we conclude that (1) baseflow MTT confirms that the hydrology of the ecosystem is dominated by shallow subsurface flow; (2) the interplay between the high storage capacity of the wet páramo soils and the slope of the catchments provides the ecosystem with high regulation capacity; and (3) MEC is an efficient predictor of MTT variability in this system of catchments with relatively homogeneous geology.

Factors influencing stream baseflow transit times in tropical montane watersheds

Abstract. Stream water mean transit time (MTT) is a fundamental hydrologic parameter that integrates the distribution of sources, flow paths, and storages present in catchments. However, in the tropics little MTT work has been carried out, despite its usefulness for providing important information on watershed functioning at different spatial scales in (largely) ungauged basins. In particular, very few studies have quantified stream MTTs or have related these to catchment characteristics in tropical montane regions. Here we examined topographic, land use/cover and soil hydraulic controls on baseflow transit times for nested catchments (0.1–34 km2) within a humid mountainous region, underlain by volcanic soil (Andisols) in central Veracruz (eastern Mexico). We used a 2-year record of bi-weekly isotopic composition of precipitation and stream baseflow data to estimate MTT. Land use/cover and topographic parameters (catchment area and form, drainage density, slope gradient and length) were derived from geographic information system (GIS) analysis. Soil water retention characteristics, and depth and permeability of the soil–bedrock interface were obtained from intensive field measurements and laboratory analysis. Results showed that baseflow MTTs ranged between 1.2 and 2.7 years across the 12 study catchments. Overall, MTTs across scales were mainly controlled by catchment slope and the permeability observed at the soil–bedrock interface. In association with topography, catchment form and the depth to the soil–bedrock interface were also identified as important features influencing baseflow MTTs. The greatest differences in MTTs were found both within groups of small (0.1–1.5 km2) and large (14–34 km2) catchments. Interestingly, the longest stream MTTs were found in the headwater cloud forest catchments.

On the use of spring baseflow recession for a more accurate parameterization of aquifer transit time distribution functions

Abstract. Baseflow recession analysis and groundwater dating have up to now developed as two distinct branches of hydrogeology and have been used to solve entirely different problems. We show that by combining two classical models, namely the Boussinesq equation describing spring baseflow recession, and the exponential piston-flow model used in groundwater dating studies, the parameters describing the transit time distribution of an aquifer can be in some cases estimated to a far more accurate degree than with the latter alone. Under the assumption that the aquifer basis is sub-horizontal, the mean transit time of water in the saturated zone can be estimated from spring baseflow recession. This provides an independent estimate of groundwater transit time that can refine those obtained from tritium measurements. The approach is illustrated in a case study predicting atrazine concentration trend in a series of springs draining the fractured-rock aquifer known as the Luxembourg Sandstone. A transport model calibrated on tritium measurements alone predicted different times to trend reversal following the nationwide ban on atrazine in 2005 with different rates of decrease. For some of the springs, the actual time of trend reversal and the rate of change agreed extremely well with the model calibrated using both tritium measurements and the recession of spring discharge during the dry season. The agreement between predicted and observed values was however poorer for the springs displaying the most gentle recessions, possibly indicating a stronger influence of continuous groundwater recharge during the summer months.

Predicting Pesticide Attenuation in a Fractured Aquifer Using Lumped‐Parameter Models

Neighboring springs draining fractured-rock aquifers can display large differences in water quality and flow regime, depending on local variations of the connectivity and the aperture size distribution of the fracture network. Consequently, because homogeneous equivalent parameters cannot be assumed a priori for the entire regional aquifer, the vulnerability to pollution of such springs has to be studied on a case by case basis. In this paper, a simple lumped-parameter model usually applied to estimate the mean transit time of water (or tracer) is presented. The original exponential piston-flow model was modified to take land-use distribution into account and applied to predict the evolution of atrazine concentration in a series of springs draining a fractured sandstone aquifer in Luxembourg, where despite a nationwide ban in 2005, atrazine concentrations still had not begun to decrease in 2009. This persistence could be explained by exponentially distributed residence times in the aquifer, demonstrating that in some real world cases, models based on the groundwater residence time distribution can be a powerful tool for trend reversal assessments as recommended for instance by current European Union guidelines.

Occurrence and transport of pharmaceuticals in a karst groundwater system affected by domestic wastewater treatment plants

The occurrence of two pharmaceuticals, ibuprofen and diclofenac, in a vulnerable karst groundwater system was investigated. The hydrogeology of the karst system was identified by collecting 3H samples in groundwater over 27 years and by performing tracer tests. The isotopes and tracer data were interpreted by mathematical modeling to estimate the mean transit time of water and to characterize the hydrogeological flow paths in the groundwater system. By this approach, a mean 3H transit time of 4.6 years for the fissured-porous karst aquifer was determined, whereas the fast flowing water in the conduit system showed a mean transit time of days. Both pharmaceuticals which infiltrated along sinkholes and small streams into the karst system were detected in concentrations of up to approximately 1 μg/L in effluent water of the wastewater treatment plants. Diclofenac was present in most samples collected from four springs discharging the karst groundwater to the rivers Altmühl and Anlauter in concentrations between 3.6 and 15.4 ng/L. In contrast, ibuprofen was rarely detected in groundwater. The results of this study suggest that both pharmaceuticals move into the fractured system of the karst system and go into storage. Thus dilution processes are the dominant control on the concentrations of both pharmaceuticals in the fractured system, whereas biodegradation is likely less important.

Multiple isotope approach to the determination of the natural attenuation potential of a high-alpine karst system

Summary 3 H, δ 18 O and δ 2 H measurements on groundwater and chemical and isotope analyses on groundwater sulfate were used to determine the origin and mean transit times of groundwater and the sources affecting groundwater sulfate in a high-alpine karstic catchment area in southern Germany. Modelling results using a lumped parameter approach yielded mean transit times of water between 4 and 12 years for different karst springs. However, results obtained from δ 18 O and δ 2 H measurements on groundwater showed that a calculated mean transit time of 12 years for groundwater flow in one karst system can only be explained by mixing of young and old tritium-free ice and snowmelt water. Groundwater sulfate characterized by δ 34 S values of around 20‰ and δ 18 O values of ca. 12‰ in concert with sulfate concentrations of approximately 11 mg/L are probably affected by evaporites. In contrast, δ 34 S values of around 6‰ and δ 18 O values up to 9.4‰ in concert with sulfate concentrations between 1 and 7 mg/L mainly derive from atmospheric deposition. However, a δ 34 S value of −7.3‰ identified for one of the karst springs demonstrates that the oxidation of sulfide-containing minerals must be considered as an additional source, affecting sulfate concentrations in groundwater. The estimated transit time distribution of groundwater shows relatively high mean transit times between 2 and 5 years with a low contribution of very short transit times of less than 1 year. The high contribution of long mean transit times of groundwater in the alpine karst system, which represents an important drinking water resource, is an important function for drinking water supplies.

Evidence for denitrification regulated by pyrite oxidation in a heterogeneous porous groundwater system

Denitrification is an important natural attenuation process that has been observed in many fissured and porous aquifers. However, an important factor limiting denitrification in aquatic systems is the microbial availability of electron donors. Pyrite as the most abundant sulfide mineral in nature represents one of the potential electron sources for denitrifiers to reduce nitrate, but the reaction mechanisms coupling denitrification processes to pyrite oxidation are still questionable. We utilized hydrochemical data and stable isotopes of nitrate and sulfate in groundwater, isotope ratios of sulfur compounds in aquifer sediments and tritium based groundwater dating for assessing denitrification processes in a pyrite-bearing porous groundwater system. The oxic part of the aquifer with mean water transit times of approximately 60 years was characterized by nitrate concentrations of around 15 mg/l and δ 15N values were similar to those typical for nitrification. In contrast, in the anoxic part with mean water transit times of up to 100 years, low nitrate concentrations accompanied by elevated δ 15N values were observed. Furthermore, isotope data of groundwater sulfate and sulfur compounds in the aquifer sediment suggest that pyrite oxidation is the dominant source of sulfate in the aquifer. The trend of increasing δ 15N values and decreasing nitrate concentrations in concert with depleted δ 34S values of groundwater sulfate similar to δ 34S values of pyrite, FeS 2, suggests that denitrification is coupled to pyrite oxidation, particularly when water mean transit time is elevated.

Flow system dynamics and water storage of a fissured-porous karst aquifer characterized by artificial and environmental tracers

Concentration breakthrough curves obtained from a tracer test and time series of environmental tracers were analyzed to characterize slow and preferential water flow in a karst aquifer of the Franconian Alb, Germany. Tritium ( 3H) and chemical tracers (uranine, bromide, strontium) were measured during low flow conditions and a storm runoff event. The mean transit time of water along the conduits was determined using bromide. Environmental tracer data collected between 1969 and 2003 were modeled to estimate the mean transit time of 3H in the fissured-porous karst system (diffuse flow). The modelling approach was also used to estimate the water volume of the karst system and the conduits. The results suggest that the total water volume in the fissured-porous karst aquifer is in the range of 57×10 6 m 3 and approximately 6% of the total water volume is stored in the soil zone and the epikarst. The water storage capacity of the conduits seems to be of minor importance. A mean transit time of bromide in the range of 14 h was calculated for the conduit flow. The fissures and the porous rock matrix have a calculated water saturated porosity of 5.5% and a mean transit time of approximately 62 years was calculated. Thus the porous rock matrix represents the major dilution and storage zone for pollutants in the karst system.

On the theory of tracer experiments in fissured rocks with a porous matrix

A model of parallel fractures, having equal spacing and width, has been applied to tracer movement in fissured rocks with a porous matrix. The exact solution has been obtained for instantaneous injection. Graphical presentations demonstrate the applicability and the limitations of particular approximations. In short-term experiments the single fracture approximation works well because the tracer has not penetrated the matrix deeply enough to be influenced by the adjacent fractures. In high matrix porosity, diffusion into the matrix is dominant and the model may be simplified by neglecting the influence of dispersivity. In low matrix porosity, a dispersion model with no diffusion into the matrix may often yield approximately the mean transit time of water. By contrast, in long-term tracer experiments, the tracer appears to exploit the whole water volume, i.e. both the mobile fissure volume and the stagnant micropore volume. The tracer curve yields the mean transit time of tracer, which is related to the mean transit time of water by the retardation factor. This factor is expressed approximately by the ratio of total porosity to fissure porosity, provided the tracer is non-adsorbable. For adsorbable tracers the microporosity acts as a strong sink due to the large adsorption surface. This fact is most probably the cause of the discrepancies observed between 14C ages in carbonate aquifers and the stable isotope shift expected from the climate change at the end of the last glaciation. It is also shown that the 14C exchange in carbonate rocks may yield 14C ages orders of magnitude too high in comparison with conventional flow data. Examples of the interpretation of artificial tracer experiments demonstrate that the model developed works surprisingly well.