Geology topics

David P. Krabbenhoft

Publications and source records attributed to David P. Krabbenhoft.

132 records · Page 8Linked to original sources

USGS1999

Characterizing hydrology and the importance of ground-water discharge in natural and constructed wetlands

Although considered the most important component for the establishment and persistence of wetlands, hydrology has been hard to characterize and linkages between hydrology and other environmental conditions are often poorly understood. In this work, methods for characterizing a wetland’s hydrology from hydrographs were developed, and the importance of ground water to the physical and geochemical conditions in the root zone was investigated. Detailed sampling of nearly continuous hydrographs showed that sites with greater ground-water discharge had higher water tables and more stable hydrographs. Subsampling of the continuous hydrograph failed to characterize the sites correctly, even though the wetland complex is located in a strong regional ground-water-discharge area. By comparing soil-moisture-potential measurements to the water-table hydrograph at one site, we noted that the amount of root-zone saturation was not necessarily driven by the water-table hydrograph but can be a result of other soil parameters (i.e., soil texture and associated capillary fringe). Ground-water discharge was not a significant determinant of maximum or average temperatures in the root zone. High ground-water discharge was associated with earliest date of thaw and shortest period of time that the root zone was frozen, however. Finally, the direction and magnitude of shallow ground-water flow was found to affect the migration and importance of a geochemical species. Areas of higher ground-water discharge had less downward penetration of CO 2 generated in the root zone. In contrast, biotically derived CO 2 was able to penetrate the deeper ground-water system in areas of ground-water recharge. Although ground-water flows are difficult to characterize, understanding these components is critical to the success of wetland restoration and creation efforts.

Wisconsin↗

USGS1996

Mercury studies in the Florida Everglades

Public concern for wildlife and human health problems due to mercury (Hg) toxicity has increased substantially since the mid-1980's. These concerns are manifested primarily by the issuance of fish consumption advisories in the majority of U.S. states, Canada, and several European countries because of high levels of mercury in game fish. Although the precise causes for this contamination problem are not completely understood, it appears that there are both source and ecosystem-specific factors that can result in elevated levels of mercury in game fish. Because mercury is known to adverse affect the human brain and nervous system, health concerns arise when elevated concentrations of mercury are detected in game fish from ecosystems where there is subsistence level consumption of fish. In extreme cases such as the Everglades, where mercury concentrations in fish consistently exceed the Florida advisory level of 1.5 parts per million, even occasional fish consumption is not recommended. For most aquatic ecosystems, atmospheric deposition is the primary source of mercury. although there are numerous instances of geologic and anthropogenic point-source contamination. There are many sources of mercury to the atmosphere, both natural and human related. Natural sources include outgassing from the oceans, volcanoes, and natural mercury deposits. Coal combustion, waste incineration, chloralkai production, and metal processing are the dominant human-related sources to the atmosphere. In ecosystems for which atmospheric deposition is the dominant source, resulting concentrations of total mercury in water are very low, generally less than 10 nanograms per liter (ng/L). The challenge to scientists is to explain the series of processes that lead to toxic or near-toxic levels of mercury in organisms near the top of the food chain (bioaccumulation), when aqueous concentrations and source-delivery rates are so low. To understand this phenomenon adequately, scientists must apply an interdisciplinary approach wherein various components of an ecosystem (atmosphere, biota, surface water, ground water, and sediments) are studied-contemporaneously. The purpose of this fact sheet is to describe the mercury contamination problem in south Florida, and the interdisciplinary project that was assembled under the auspices of the U.S. Geological Survey South Florida Ecosystem Program to investigate the underlying processes that cause mercury bioaccumulation.

Florida↗

USGS1996

Groundwater inflow measurements in wetland systems

Our current understanding of wetlands is insufficient to assess the effects of past and future wetland loss. While knowledge of wetland hydrology is crucial, groundwater flows are often neglected or uncertain. In this paper, groundwater inflows were estimated in wetlands in southwestern Wisconsin using traditional Darcy's law calculations and three independent methods that included (1) stable isotope mass balances, (2) temperature profile modeling, and (3) numerical water balance modeling techniques. Inflows calculated using Darcy's law were lower than inflows estimated using the other approaches and ranged from 0.02 to 0.3 cm/d. Estimates obtained using the other methods generally were higher (0.1 to 1.1 cm/d) and showed similar spatial trends. An areal map of groundwater flux generated by the water balance model demonstrated that areas of both recharge and discharge exist in what is considered a regional discharge area. While each method has strengths and weaknesses, the use of more than one method can reduce uncertainty in the estimates.

Water Resources Research↗

USGS1995

Mercury contamination of aquatic ecosystems

Mercury has been well known as an environmental pollutant for several decades. As early as the 1950's it was established that emissions of mercury to the environment could have serious effects on human health. These early studies demonstrated that fish and other wildlife from various ecosystems commonly attain mercury levels of toxicological concern when directly affected by mercury-containing emissions from human-related activities. Human health concerns arise when fish and wildlife from these ecosystems are consumed by humans. During the past decade, a new trend has emerged with regard to mercury pollution. Investigations initiated in the late 1980's in the northern-tier states of the U.S., Canada, and Nordic countries found that fish, mainly from nutrient-poor lakes and often in very remote areas, commonly have high levels of mercury. More recent fish sampling surveys in other regions of the U.S. have shown widespread mercury contamination in streams, wet-lands, reservoirs, and lakes. To date, 33 states have issued fish consumption advisories because of mercury contamination. These continental to global scale occurrences of mercury contamination cannot be linked to individual emissions of mercury, but instead are due to widespread air pollution. When scientists measure mercury levels in air and surface water, however, the observed levels are extraordinarily low. In fact, scientists have to take extreme precautions to avoid direct contact with water samples or sample containers, to avert sample contamination (Fig 3). Herein lies an apparent discrepancy: Why do fish from some remote areas have elevated mercury concentrations, when contamination levels in the environment are so low?

Fact Sheet↗

USGS1995

Applications of isotopes to tracing sources of solutes and water in shallow systems

New awareness of the potential danger to water supplies posed by the use of agricultural chemicals has focused attention on the nature of groundwater recharge and the mobility of various solutes, especially nitrate and pesticides, in shallow systems. A better understanding of hydrologic flowpaths and solute sources is required to determine the potential impact of sources of contamination on water supplies, to develop management practices for preserving water quality, and to develop remediation plans for sites that are already contaminated. In many cases, environmental isotopes can be employed as 'surgical tools' for answering very specific questions about water and solute sources. Isotopic data can often provide more accurate information about the system than hydrologic measurements or complicated hydrologic models. This note focuses on practical and cost-effective examples of how naturally-occurring isotopes can be used to track water and solutes as they move through shallow systems.

Conference Paper↗

USGS1992

Water, Energy, and Biogeochemical Budgets (WEBB) program: Data availability and research at the Northern Temperate Lakes site in north-central Wisconsin

More than three thousand kettle lakes, widely dispersed within a mixed temperate forest ecosystem, are predominant features of the Northern Highland area of north-central Wisconsin. A hydrological and biogeochemical investigation of seven of these lakes and their watershed area is currently in progress as part of the Water, Energy, and Biogeochemical Budgets (WEBB) program of the U.S. Geological Survey. The objectives of the overall WEBB program are to: (1) improve understanding of processes controlling water, energy, and biogeochemical fluxes in freshwater systems, the interactions among those processes, and their relations to climatic variables; and (2) improve the capability to predict freshwater, energy, and biogeochemical budgets over a range of spatial and temporal scales. The focus of the WEBB project in Wisconsin is hydrologic research in the Northern Temperate Lakes (NTL) area of the state. Based on the premise that an understanding of hydrologic and biogeochemical cycles in the watershed is predicated on a thorough understanding of the individual components that control the water flow, the project involves a detailed study of the hydrologic budget, including the roles of rainfall, streamflow, ground water, and flow in the unsaturated zone. It also involves investigation of ground-water/ surface-water interactions. The objectives of the Northern Temperate Lakes WEBB project are to: (1) describe processes controlling water and solute fluxes in the NTL watersheds; (2) examine interactions among those processes and their relations to climatic variables; and (3) improve the capability to predict changes in water and solute fluxes for a range of spatial and temporal scales. A variety of procedures will be used to address the objectives of the project including: installation of piezometers, lysimeters, stream-gaging sites, precipitation collectors, climate-monitoring instruments; analysis of transport of nutrients and other solutes through the system; analysis of changes in isotopic composition of water; and temperature profiling. The implementation of these procedures is described in this report. The NTL-WEBB study area includes seven lakes that are also the site of a Long-Term Ecological Research (LTER) project, sponsored by the National Science Foundation. This project incorporates diverse research investigations conducted by faculty and research associates of the University of Wisconsin-Madison. The research orientation of NTL-LTER is principally toward aquatic ecology and geochemistry of the lakes. The WEBB research plan, with its emphasis on hydrologic processes in the lake watersheds, is designed to complement and enhance the LTER work.

Wisconsin↗