Conduit Flow Systems Research Articles

Proposed method for groundwater vulnerability mapping in carbonate (karstic) aquifers: the COP method

The ‘COP method’ has been developed for the assessment of intrinsic vulnerability of carbonate aquifers in the frame of the European COST Action 620. This method uses the properties of overlying layers above the water table (O factor), the concentration of flow (C factor) and precipitation (P factor) over the aquifer, as the parameters to assess the intrinsic vulnerability of groundwater. This method considers karst characteristics, such as the presence of swallow holes (C factor) and their catchment areas as well as karstic landforms, as factors which decrease the natural protection provided by overlying layers (O factor). The P factor allows for consideration of the spatial and temporal variability of precipitation, which is considered the transport agent of contamination. Two carbonate aquifers in the South of Spain, Sierra de Libar (a conduit flow system) and Torremolinos (a diffuse flow system), have been selected for the application and validation of the method and the results have been compared with three methods widely applied in different aquifers around the world (AVI, GOD and DRASTIC). Comparisons with these methods and validation tools (hydrogeological data and tracer test) show the advantages of the COP method in the assessment of vulnerability of karstic groundwaters.

Influence of rainfall quantity on the isotopic composition ( 18O and 2H) of water in mountainous areas. Application for groundwater research in the Yunquera-Nieves karst aquifers (S Spain)

The isotopic composition ( 18O and 2H) of rainwater and groundwater was determined in the Yunquera-Nieves hydrogeological unit, a karstic massif in S Spain with complex orography and hydrogeology. The aims were to identify: (1) the source of the water, (2) the main factors determining the isotopic content, (3) the catchment area of the springs and (4) the aquifer behaviour. The isotopic content of the sampled waters is of mixed Atlantic–Mediterranean origin and is opposite to the quantity of rainwater distribution, both in space and time. Spatially, water is isotopically less depleted toward the eastern sector, where rainwater quantity is lower (which is locally affected by the orography): thus, it is possible to differentiate the catchment areas of the main springs. Aquifer recharge is produced by winter precipitation, when rainfall water is isotopically more negative and evaporation is lower. The decrease in rainfall quantity during the study period provoked an increase in δ 18O and δ 2H values, both in rainwater and in groundwater. The recorded variations of the isotopic content of the rainfall are rapidly evident in the spring waters, which demonstrates a well developed karst network inside the carbonate aquifers, and thus the existence of conduit flow systems.

Use of hydrodynamic and hydrochemistry to characterise carbonate aquifers. Case study of the Blanca-Mijas unit (Málaga, southern Spain)

To validate the geological and piezomet- ric subdivision of two carbonate massifs in southern Spain, and to carry out a characterisation of aquifer behaviour, 23 representative springs and wells were monitored at the end of the dry season. Tempera- ture, pH and electrical conductivity and major ions were measured. Calcite (SIc) and dolomite (SId) saturation indexes, PCO2 and cation exchange index (CEI) were calculated with the SOLUTEQ code. Water flow and electrical conductivity were also monitored at the eight main springs. Sierra Blanca and Sierra Mijas constitute an important carbonate hydrogeological unit in southern Spain, supplying water to 250,000 people on the Costa del Sol. Hy- drochemically, two basic types of aquifer behaviour can be distinguished: conduit and diffuse flow sys- tems. The former are found in the calcareous mar- bles of the western Sierra Blanca. They present a calcium-bicarbonate hydrochemical facies with low mineralization, and with quick outflow and water chemistry variations in response to rainfall; hydro- chemical time series present frequency distributions with several modes. The diffuse flow systems are characteristic of the dolomitic marbles of eastern Sierra Blanca and Sierra Mijas. These waters are more highly mineralised and present a calcium- magnesium bicarbonate facies. They are character- ised by slight discharge fluctuations, with nearly unimodal frequency distributions. Aquifer beha- viour affects groundwater exploitation in terms of the success of boreholes and the specific yield of the pumping wells, which are less in the conduit flow systems in western Sierra Blanca.

Derivation of effective hydraulic parameters of a Karst Aquifer from discharge hydrograph analysis

In well‐developed karst terrains, three or more distinct portions of the karst continuum can be identified from hydrographs of springs issuing from the karst aquifer. Hydrographs from two karst springs within the same drainage basin at the Crane Naval Surface Warfare Center, Indiana, have been analyzed, and ratios of transmissivity and specific yield (T/Sy) have been established for the conduit and diffuse flow systems. These ratios have been compared with values of T derived from aquifer tests, so that independent values of Sy can be calculated for the diffuse system. Similarly, if the value of Sy is assumed to be 1.0 for a pure conduit, then independent values of T can be calculated for this end‐member of the karst continuum. The values of T and Sy derived from this study are similar to values obtained from a dye trace of the conduit‐dominated flow system and of values derived from aquifer tests of the diffuse flow system. Values of T for the conduit system of these springs may need to be established at a local scale, while the values for the diffuse flow system may be applicable at a regional scale. A hydrograph separation using isotopic data suggests that the intermediate‐flow system represents a mix of water from the conduit and diffuse flow systems. If this portion of the hydrograph is a truly mixing phenomena, ratios of T/Sy cannot be determined from the hydrograph analysis presented herein. However, if instead, the intermediate‐flow system represents water released from a third reservoir (such as small fractures), ratios of T/Sy can be established for the intermediate‐flow system.

Application of geochemistry and radioactivity in the hydrogeological investigation of carbonate aquifers (Sierras Blanca and Mijas, southern Spain)

The chemical characteristics, 3H contents and radioactivity of groundwaters from the Sierras Blanca and Mijas (Southern Spain) have been studied in relation to the chemical composition and radioactivity of the aquifer host rocks, and the residence time of the water. The Sierras Blanca and Mijas are made up of calcitic and dolomitic marbles of Triassic age. The groundwaters that drain the calcitic marbles (which outcrop principally in the western Sierra Blanca) have less mineralization, which descreases quickly with recharge (as does the 3H content), and the gross alpha and beta activities are below detection limit. This is due to the short residence time of water inside the aquifers which are conduit flow systems. The waters of the dolomitic marbles (eastern Sierra Blanca and Sierra Mijas) have higher and less variable mineralization and contain greater concentrations of Mg 2+, SiO 2 and SO 2− 4 (ions normally associated with slow flows). The 3H contents are more uniform with time (indicating an older age) and there is detectable natural radioactivity, because the waters have a longer residence time in the aquifers, which are diffuse flow systems.

Modeling of storm responses in conduit flow aquifers with reservoirs

In aquifers containing large voids, such as karst aquifers with caves or basaltic aquifers with lava tubes, hydrographs at wells or springs are used to analyze the structure and response of the hydrogeological system. Numerical modeling of hydrograph response is commonly based on either inverse techniques or postulated flow geometries. However, the range of mechanisms for generating hydrograph responses have not been fully investigated. Physical modeling of these complex non-Darcian systems permits better understanding of the storm responses that conduit systems may generate. Using a numerical model of conduit flow systems which incorporates turbulent flow, some of the mechanisms that can alter storm pulses were investigated by treating them as combinations of pipes that connect reservoirs. The results indicate that the response of a conduit-flow aquifer can range from what has been called ‘diffuse’ or ‘steady’ to ‘conduit’ or ‘flashy’, without employing a diffusive component. A full range of behavior can be the result of changes from phreatic to epiphreatic conditions in a conduit, changes in conduit geometry, or multiple springs draining the same system. The results provide a quantitative tool to assess spring and well hydrographs, and illustrate mechanisms that can generate observed responses, which have previously been qualitatively interpreted.