Carol M. Wicks

Modeling of storm responses in conduit flow aquifers with reservoirs

Abstract 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.

Physical response of a karst drainage basin to flood pulses : example of the Devil's Icebox cave system (Missouri, USA)

Abstract In karst aquifers, water moves from the recharge area (sinkhole plains and swallets) to the discharge area (springs), traveling kilometers through the groundwater system in a period of hours to days. Transit times through karst aquifers are a function of the conduit geometry and connectedness, intensity and duration of the recharge event, and antecedent soil moisture. Often many of these factors are unknown or difficult to quantify. Therefore, predicting the response of a karst basin to recharge is difficult. Numerous researchers have attempted to understand the response of a karst basin, but a good understanding of whether the response is dependent on local features or regional effects is currently lacking. From April 1994 to May 1995, flood pulse hydrographs from a karst aquifer with well-developed and well-documented conduits (Devils Icebox cave system) were obtained from a gaging station near the spring of the karst basin. Data were also collected from within the conduit system in an attempt to determine whether flow was locally controlled by constrictions in the conduits. Based on an application of Bernoullis equation, analyses of the changes in kinetic head and potential head over time indicated local control during storm events. The observed sediment patterns and water level variations also support localized flow control during storm events. A numerical model of the constrictions was tested and reproduced the responses observed at the spring during initial periods of storm events. The model illustrated that the constricted flow was very sensitive to recharge. It also illustrated the transition from local control due to constriction to regional controls due to the aquifer matrix.

Regional Hydrogeochemistry of a Modern Coastal Mixing Zone

In west central Florida, groundwater samples were collected along flow paths in the unconfined upper Floridan aquifer that cross the inland, freshwater recharge area and the coastal discharge area. A groundwater flow and solute transport model was used to evaluate groundwater flow and mixing of fresh and saline groundwater along a cross section of the unconfined upper Floridan aquifer. Results show that between 8% and 15% of the fresh and 30–31% of the saline groundwater penetrates to the depth in the flow system where contact with and dissolution of gypsum is likely. The deeply circulating fresh and saline groundwater returns to the near-surface environment discharging CaSO4-rich water to the coastal area where it mixes with fresh CaHCO3 groundwater, resulting in a prediction of calcite precipitation in the modern mixing zone.

How hydrogeology has shaped the ecology of Missouri’s Ozark cavefish, Amblyopsis rosae, and southern cavefish, Typhlichthys subterraneus: insights on the sightless from understanding the underground

Two troglobitic fishes of conservation concern that inhabit Missouri are the Ozark and southern cavefishes, Amblyopsis rosae and Typhlichthys subterraneus, respectively. These species inhabit the groundwater of karstified bedrock in the Springfield and Salem plateau regions of the state, respectively. These two areas differ substantially – geographically, geologically, and hydrologically. This paper explores how these differences interact in shaping (1) the habitat in which these two species dwell, (2) the resulting ecological constraints that the fishes face, (3) how their ecologies and populations reflect these constraints, and (4) how conservation and management efforts may need to be tailored to best compliment the unique challenges that each species presents.

Thermal Variations in the Hyporheic Zone of a Karst Stream

This investigation quantifies how temperatures in the streambed sediments of a karst stream fluctuate in relation to discharge, seasonal, and diurnal temperature variations as the stream passes through a karst window. Furthermore, the linkages between meteorological processes and temperature variation in the hyporheic zone are delineated. Examination of a high-resolution, three dimensional record of temperature variation in a karst stream substrate provides insight into thermal disturbances in the hyporheic zone. Temperatures in the upper portion of the hyporheic zone are strongly linked to air temperatures via the surface water. The variation is considerably less as depth increases. The annual temperature variation in the lower portion of the shallow hyporheic zone is reduced by one-third, relative to the variation observed in the surface water and upper substrate. During storm events, the upper portion (0-5 cm) of the shallow hyporheic zone is subject to a thermal regime very similar to surface stream water. However, below these depths the sharpest temperature fluctuations are effectively muted within the stream substrate. More frequent diurnal variations, particularly those associated with baseflow conditions, are also dampened within the substrate.

Persistence of 17 β-Estradiol in Water and Sediment-Pore Water from Cave Streams in Central Missouri

Concentrations less than 10 ng/L of 17 β-estradiol (E2), a natural estrogen, have been linked to adverse health effects in fish, including skewed sex distributions, reproductive failure, and organ impairment. The persistence of E2 in carbonate aquifer systems is not well documented. Water and sediment from cave streams within the Ozark Plateau of Missouri, USA, were collected and analyzed for E2. The persistence of E2 in the water was examined in two separate experiments, in which the holding temperatures (20°C vs. 4°C), bottle type, exposure to light, and filtration were varied. During two trials, no statistical difference was observed in the concentration of E2, suggesting that E2 is stable within the water. The fate of E2 was also examined in sediment-pore water collected from the cave streams in two independent trials. In trial 1, a significant decrease in E2 was noted over the 29 days of the experiment. However, in trial 2, no change in E2 concentration was observed. The results indicate that E2 is relatively stable in cave stream water and may persist in the sediment.

Genetics and Speleogenesis

Karst environments are ideal places to study the linkages between biological and geological processes because of the evolutionary record in the DNA of the resident species and because of the remarkable historical record in the rocks and sediments. The trace of time is apparent in karst, yet time is seldom discussed in and of itself bykarst scientists. The Karst Research Institute (KRI, part of the Scientific Research Centre of the Slovenian Academy of Sciences (ZRC-SAZU)) and the Karst Waters Institute (KWI, Leesburg, Va.) hosted a meeting to specifically discuss “time in karst.” The KWI and the KRI have consistently provided outlets for interdisciplinary research efforts. This jointly hosted meeting brought together about 130 scientists from 27 countries in Postojna, Slovenia. The scientists included 40 who were supported by Marie Curie Grants for Young Researchers. The technical sessions focused on the topics paleokarst, historical biogeography, agedating methods, the sediment record, the age of karst landscapes, and the age of animal lineages. A proceedings volume is available elsewhere (www.karstwaters.org/publications/).