Living With Karst: A Fragile Foundation
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Karst geomorphology. Basil Blackwell, Oxford, United Kingdom. Veni, G. DuChene, N. Crawford, C. Groves, G. Huppert, E. Kastning, R. Olson, and B. Groundwater basins are flow-paths through which previously surface drainage moves to its point of resurgence and discharge. The boundaries and direction of the groundwater basin and the relic surface basin can be the same where the underground drainage has merely replaced surface flow. However, underground conduit systems are often more complex.
They may have more than one channel carrying water and they may have branching such that water from one drainage basin discharges at more than one spring.
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For these reasons karst aquifers are not well understood. It is thought that water flows through porous surface media to the vadose zone or enters the vadose zone through swallow holes and conduits. Water is stored below these conduits in the phreatic zone and beside the conduits in non-flow conduits and caves. Phreatic storage further.
Unlike non-karst areas there exists a great potential for mixing of drainage and storage waters as flow is sometimes flashy and turbulent. Above is a diagram from White, With this potential for mixing comes greater possibilities for the transmission of contaminants White, In many carbonate aquifers the flow type varies between diffuse and conduit.
Diffuse flow occurs in tight fractures, joints, and bedding planes. Such flow, where velocities are low, is said to be laminar not turbulent and where no mixing occurs. Williams, Storage can even take place in this "subcutaneous" zone when a perched water body develops because of seasonal saturation of the lower permeability underlying rocks. With such rapid subsurface water entry and all this connectivity and mixing its easy to see why groundwater contamination is a problem.
But, groundwater pollution is not the only degradation of karst systems. In an introductory article in a special supplement to Catena Paul Williams classifies human-induced impacts on karst terrains in the following categories:. The current conflict in Yugoslavia is occurring in a region that is highly karstified.
Caves and Karst
At one point on NPR I heard some military talking head mention some of the obstacles to troop movement in the area and he mentioned the "difficult terrain. But, I digress…. Caves provide natural shelter and cave occupation, though dating back to mid Pleistocene, is still practiced today in areas of Southern Europe and of china. Deforestation has a profound indirect impact and dates back to years ago in the northern Mediterranean basin. In the Greek and Roman eras deforestation stimulated the erosion of hillsides and sedimentation of the Mediterranean valleys.
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The loss of trees increases runoff. In many karst areas soils are poor and thin and runoff can expose bedrock outcroppings — leaving little left for cultivation. Karst areas that are cleared for agriculture are prone to soil loss in sinkholes and swalletts. Terracing of hillsides as is done in China sometimes helps stem the tide, but more often than not agriculture results in desertification.
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The over-tapping of groundwater supplies results in a lowing of the water table and facilitates sinkhole collapse and other surface collapse. Limestone and marble have been mined for building and for sculpture "since the minoan age of Crete"! The most common problems associated with urbanization in karst area are flooding, pollution and groundwater collapse. Increased demand for water depletes aquifers, impervious surfaces concentrate runoff, and construction of large buildings on unstable ground is hazardous to people and to karst. In the remaining portion of this paper, I want to focus on two specific issues: groundwater contamination and to a degree, agricultural activities as a source of groundwater impacts and sinkhole collapse.
According to geomorphologists and karst hydrologists, like Nicholas Crawford, shallow aquifers in karst areas are probably the most vulnerable in the world to groundwater contamination. Karst aquifers receive both diffuse recharge from percolation through the soil and concentrated recharge from surface runoff that flows directly into the aquifer at stream sinks and sinkhole drains. Crawford and Whallon, Because of the rapid velocities of these underground streams, contaminants may travel several miles through the aquifer in only a few hours. Contaminants from agricultural activities, such as nitrates, bacteria from livestock waste, and pesticides, are potential problems in karst terrain.
Contaminants found in urban storm water runoff such as: lead, chromium, oil and grease, petroleum products, solvents, and bacteria from pet and animal wastes may be a threat to people using karst water supplies and to cave life. Additionally, it has been shown that septic tank effluent can travel through the thin soils which are characteristic of most karst areas into the aquifer and then to a spring in only a few hours. They assert that predicting flow response is not possible on the individual conduit level. Rather, they propose that modeling transport is much more accurate if it is done on a "basin scale" perspective, because the hydraulic geometry is exceedingly complex and transport depends on volume, shape, location of conduits, position of conduits relative to the water table and the degree of development of other karst features, like sinkholes and losing streams.
Hoke and Wicks suggest that using a linear systems approach is useful because it describes basin transport in terms of the distribution of travel or as a residence time -- and without having to have detailed knowledge of internal structure. In the linear systems model each basin has a unique "kernal function" which relates recharge over a spatial area.
Kernal functions can be derived through dye tracing methods and takes into account the method by which contaminants enter the cost system. For example, a spatially and uniformly distributed non-point source contaminant to the basin will exhibit a different response than a discrete point source contaminant. Their model used precipitation, spring discharge, and recharge over a spatial area as the variables. Testing of the model yielded predictable results.
Transport models are useful in that, if accurate, they can be generalized to basins of similar character. They begin the article by stating that many remediation efforts in karst areas have been unsuccessful, particularly those that rely on groundwater extraction for cleanup.
They have failed because the source, storage and transport characteristics are different for karst than for other aquifers. Instead, understanding of the complex nature of karst storage and transport is necessary for toxic cleanup. They even admit that with some contaminants, in some settings — cleanup may not be possible. Remediation in karst aquifers includes proper site characterization, and an understanding of contaminant transport mechanisms. Site characterization means comprehensive surveys and dye tracings that monitor all springs and seeps in the area as potential areas of resurgence -- not ones that monitor "downstream" only.
In addition, they note the importance of monitoring water chemistry at different times of the year to determine if recharge comes from bedrock or subsurface waters. The characteristics of contaminant transport are important as well. Different types of contaminants have different flow characteristics.
L-NAPL light non-aqueous phase liquid organic compounds like petroleum are hydrophobic and tend not to be flushed out.https://swinidmydarkwa.cf
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Rather, because they float on top, they are absorbed and remain in the "interstitial spaces of the aquifer matrix" Pg. Plumes from these concentrated sources of toxin continue to contaminate the aquifer. Knowing the transport and "resting" characteristics of contaminants can aid in locating them in the aquifer so that they might be removed.
But Barner and Uhlman conclude by saying that many times the source of contamination cannot be removed, for physical or economic reasons.
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When this is the case, they suggest implementing groundwater filtration technologies at the spring resurgence instead. Leaks, spills or deliberate dumping of toxic or explosive chemicals are a particularly serious hazard in karst areas. According to Crawford, contamination problems are aggravated in karst areas by the practice of disposing of solid and liquid wastes in sinkholes where they are prone to being washed directly into the aquifer.