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Karst Aquifer Tests Karst Karst Aquifer Tests Karst • The term karst is derived from the Slovenian word kras, which is the name of a mountain range on the border between Slovenia and Italy. The term karst is most often applied to the distinctive landforms found on limestone and dolostone bedrock. These include sinking streams (streams which disappear underground into sinkholes or caves), caves, karren (bedrock that has been sculpted by solution), and dolines or sinkholes (round depressions). Aquifer Type Aquifer Porous Media Fractured Karst Characteristics Rock Permeability Mostly 10 Mostly 20 Almost All 20 Flow Slow, laminar Possibly Likely fast/turbulent fast/turbulent Isotropy Most iso Less iso Highly aniso Homogeneity Most homo Less homo Non-homo Flow Predictions Darcy flow Darcy flow may Darcy rarely not apply applies Storage Saturated zone Saturated Zone Sat/unsat zone (epikarst) Head Variation Minimal More May be extreme Chemistry Min variation More variation May be extreme A Hydrogeologist’s View (NRC): • “It all comes down to a matter of scale. A fractured or dissolutioned aquifer will respond to pumping stress the same way as a porous aquifer, if the scale is large enough. For instance, if you're looking at a water supply system you can run a long-term (a week or so) aquifer test and analyze it like it's a porous aquifer, because the time scale is sufficiently large. That is one of the simplifing assumptions that goes into a pumping test. The longer the test is run the more it will look like a garden variety aquifer.” …more (NRC) • The anisotopy can be measured, using Hantush or Papadopulos, which will give you a major transmissivity direction and a minor transmissivity direction in an elipsoid. The major T will be the main dissolution channels. Csallany (Dolomite/limestone) • “…the controlled pumping test method may describe the hydraulic properties of the dolomite and limestone on an areal basis, but does not accurately describe the drawdown in the immediate vicinity of a pumped well.” • Analysis as leaky confined aquifer (Hantush/Jacob) • Response in distant wells Taylor and Greene, USGS: • “The major difficulty facing the hydrologist is that karst aquifers typically exhibit dual ground-water flow regimes, that is, fast (conduit-dominated) flow and slow (diffuse) flow. …more (Taylor and Greene) • Quantitative water-tracing tests – dye tracers (QTRACER) • Traditional aquifer tests – dual porosity models (larger conduits dominate early in test, diffuse flow more important later in test) Worthington – Buried Karst • The order of magnitude differences between pump, slug and packer tests • Presence of water table troughs • Rapid water level response following recharge events …more (Worthington) • Rapid change in water quality following recharge events • Water undersaturated with respect to calcite following recharge events • Wide range in fracture apertures along major bedding planes Debieche et al.: • Inferring behavior from long-term borehole data • Example, 5 years of data; water level versus cumulative yield • 80% of data explained by linear function; only floods following heavy storms and non-pumping cannot be taken in to account …more (Debieche) • 2 slopes: recharge and draining part of the aquifer fractures • Hydraulic behavior of the aquifer differs according to the pumping rate: equivalent continuous medium at low rate, dual porosity at high rate Shevenell: Well Hydrographs • Shape of rising limb largely dictated by character of storm event • Recession limb – 2 or more limbs – Fast response to conduit flow – Slower responses owing to flow through fractured and unfractured porous media • Short, intense storm most useful … more (Shevenell) t Q Q0e Where ε is the exhaustion coefficient (recession slope) … more (Shevenell) In well-developed karst, 3 segments with different slopes occur 1. 1st/steepest: drainage of karst (predominately) 2. 2nd/intermediate: emptying of well- connected fractures 3. 3rd/least steep: matrix drainage … more (Shevenell) Each slope has a characteristic λ for any storm: ln(Y / Y ) ln(Q / Q ) 1 2 1 2 t2 t1 t2 t1 Where Y’s are water levels and Q’s are flows at times t … more (Shevenell) Where: Q1 to Q2 represent conduit-dominated flow/drainage Q2 to Q3 represent fracture dominated flow Q3 to Q4 represent matrix flow … more (Shevenell) Baseflow relation to groundwater storage V1 Q1(t2 t1) / ln(Q1 / Q2 ) Vt Qt / Where V is the volume of water in storage at time t; Q is flow rate at time t … more (Shevenell) (V1 V2 ) (Q1 Q2 ) / (V1 V2 ) AS y (Y1 Y2 ) AS y1 AS y2 AS y3 X1, X 2, X 3, Q1 Q1 Q1 X1 X 2 X 3 … more (Shevenell) A (area) assumed constant, Sy’s estimated for each segment T estimated: T 1.071 log(Q / Q ) (t t ) 1 2 S 2 1 L2 L is distance from discharge to gw divide; S = Sy for unconfined aquifer … more (Shevenell) Limitations: 1. Sharp storm pulses best 2. Recession curves should be complete (for matrix estimates) 3. log of WL used, relative elevation used 4. Area for conduit, fracture, and matrix flow assumed same Literature Cited • Csallany, Sandor C. The hydraulic properties and yields • Debieche, T.H., Y. Guglielmi, and J. Mudry. 2002. Modeling the hydraulical behavior of a fissured-karstic aquifer in exploitation conditions. J of Hydrology 257:247-255 • Shevenell, Lisa. 1996. Analysis of well hydrographs in a karst aquifer: estimates of specific yields and continuum transmissivities. J of Hydrology 174:331-355 • Taylor, Charles, and Earl Greene. 2001. Quantitative approaches in characterizing karst aquifers. In Eve L. Kuniansky, Editor. USGS Karst Interest Group Proceedings, WRIR 01-4011, p. 164-166. • Worthington, Stephen. 2002. Test methods for characterizing contaminant transport in a glaciated carbonate aquifer. Environmental Geology 42:546-551.
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