Groundwater Fluxes Across Interfaces (2004) / Chapter Skim
Currently Skimming:
2 Diffuse and Focused Recharge and Discharge
2 Diffuse and Focused Recharge and Discharge
Pages 16-31
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 16... ...
The spatial distribution of recharge fluxes and the travel time for movement of water from the land surface to the water table largely determine the vulnerability of aquifers to contamination including agricultural contaminants (e.g., BohIke, 2002; Fogg et al., 1999) and are used to define zones of capture for pumping wells in welThead protection programs (Reilly and Pollock 1993~.
|
From page 17... ...
. EXPERIMENTAL BENCHMARK SITES In effect there are presently a large number of generally uncontrolled recharge and/or discharge experiments taking place each year, as hundreds of researchers, practitioners, and managers collect and analyze data, or build models of systems, in which groundwater recharge and discharge are quantified.
|
From page 18... ...
One promising way to achieve consistency in addressing recharge and discharge issues would be to establish a network of experimental benchmark sites that sample a wide range of landscape and geohydrologic types, and climatic regimes, building on the concept of hydrogeologic conceptual models that is fundamental to hydrogeologic analysis. An example of a framework for organizing hydrogeologic conceptual models or generic hydrogeolog~c settings is the Fundamental Hydrologic Landscape Unit (FHEU)
|
From page 19... ...
The following sections illustrate how benchmark sites can be used to conduct research on recharge and discharge issues and methods by focusing attention on four generic hydrogeologic settings: karst, glaciated Midwest, mountain and valley, and coastaVestuarine. These settings represent four different types of landscapes and hydrogeologic conditions, and are used to suggest specific focal areas for research at each type of site.
|
From page 20... ...
Remote sensing of vegetation and soil water content, geomorphological analysis, and field observation and mapping of focused recharge and discharge relative to fracture systems over the watershed would be a first step towards evaluating recharge and discharge in the karst setting. Numerous studies of karst hydrology in many climatic settings show that diffuse recharge and discharge is a very small component of a karst water budget (White, 1988~.
|
From page 21... ...
Generally, late fall rains and spring snowbelt events provide the majority of the annual recharge. Focused recharge often occurs after extended periods of wetness such as snowmelt or a series of intense rains.
|
From page 22... ...
~ 2001 by American Water Resources Association. mated from discharge measurements.
|
From page 23... ...
Diffuse and Focused Recharge and Discharge 23 BOX 2-1 Horizontal Variation in Geochemistry of Focused Discharge into a Regional Spring Complex One might assume that in areas of focused groundwater discharge relatively few samples would be needed to characterize water chemistry because focused discharge consists of an integrated, well mixed water volume channeled from a large collection area. However, this may not be true.
|
From page 24... ...
, and was generally larger than the horizontal variability. The vertical variability was also larger than the temporal variability over the growing season.
|
From page 25... ...
The mountainous portions (uplands) are often rugged and are composed of exposed rock or may have a weathered zone of a few tens of meters and be covered with vegetation, making direct measurements of groundwater recharge difficult (Forster and Smith, 198Sa)
|
From page 26... ...
. 2001 by American Water Resources Association.
|
From page 27... ...
Freshwater seepage below the high tide line is called submarine groundwater discharge or SGD (e.g., Burnett et al., 2002; Taniguchi et al., 2002~. Two factors greatly complicate the analysis of g~oundwater discharge in the immediate vicinity of the coastline: the position of the salt water interface influences the location of discharge sites, and tidal fluctuations cause reversals in the groundwater flow direction.
|
From page 28... ...
ocean 7 DRIVING FORCES: c = con vectl'on h _ hydraulic head t = tidal pumping W = wave sef-oP c ~ Cafe t ~ I _ ~ ~ ~ ~ ~ ~ ''brx~7 wamr ~ — t ~` at; ~'%`' ~ ~'~'~''~,~, ,:~`'~",~'~'`""bra~'sh watery, water table ;7 fresh water ~ unconfined aquifer ~ ., confine ag~ffer fresh water ~ FIGURE 2-7 = Schematic depiction of processes associated with submarine groundwater discharge.
|
From page 29... ...
There was also reasonable consistency between the estimates of SGD derived by integrating the point estimates of SGD from the seepage meters across a zone extending 200 m offshore, with the estimates derived from geochemical tracers. Predictions of SGD using a density-dependent groundwater flow model yielded values for the offshore discharge that were approximately one order of magnitude smaller than these estimates.
|
From page 30... ...
Driving forces originate both on land (e.g., fluid pressures beneath the sea bed that exceed the seafloor hydrostat, or a response to a local precipitation event in an unconfined system) , and in the ocean (wave action, diurnal and spring / neap tides)
|
From page 31... ...
Other, still ill defined processes may also be involved. At Waquoit Bay on Cape Cod Massachusetts, measurements of seepage fluxes using a large array of seepage meters yielded a pattern of discharge that could not be explained with simple hydrologic models (Michael et al., 20031.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.