Recharge area

Next we look at how temperature gradients along an aquifer might affect the silica content of flowing groundwater. We consider a symmetrical aquifer that descends from a recharge area at the surface to a depth of about 2 km and then ascends to a discharge area. Temperature in the calculation varies linearly from 20 °C at the surface to 80 °C at the aquifer s maximum depth. 

Figure 4.8 shows an example of the estimation of paleotemperature based on noble gas measurements in the groundwater of some aquifers in Texas (State et al., 1992). Paleotemperatures estimated from the noble gas concentration shows a large drop (about 5.2°C) at the distance 30km away from the recharge area [Figure 4.8(c)], which corresponds to an age between 12,000 and 17,000 years, close to the last glacial maximum (about 18,000 years ago). 

Figure 7.42. Comparison between (A) an idealized plot of variation in 8180 and 813C for carbonates subjected to vadose and phreatic meteoric diagenesis (after Lohmann, 1988) with (B) the meteoric alteration trend observed for the Key Largo Limestone, Florida, U.S.A. (after Martin et al., 1986). The critical trend in isotopic composition is termed the meteoric calcite line. This trend may be modified at the water recharge surface where evaporation is an important process, caliche is formed and the diagenetic phases are depleted in 13C derived from soil-gas CO2. Another modification can occur distally to the recharge area where precipitating carbonate cements may have isotopic ratios nearly equivalent to dissolving phases.

Figure 12.1 Map showing locations of sites (squares) used for the soil profiles and boreholes (C and B), and contents of arsenic in the leach fractions of the top layer of the soil profiles. Squares with stars denote samples used for the Pb isotope study other squares are locations of profiles used in a geochemical study (Ayuso, unpublished data). Dashed lines enclose areas containing wells characterized by Lipfert and Reeve (2004) and Lipfert et al. (2007) as containing high-arsenic groundwater (As >1.3 pmol L ), medium arsenic groundwater, and low arsenic groundwater in the Mount Percival recharge area (most wells have <0.13 jimol L-1). Solid line encloses the drainage basin in this study.

Fig. 9.17 Altitude effect, reflected in groundwater of known recharge areas, Nicaragua. An effect of —0.26%o c>180/100m is obtained, the same as observed for the regional precipitation (see Fig. 9.16). (From Payne and Yurtsever, 1974.)...

Regional aquifers consist, in certain cases, of a recharged outcrop of a conductive rock layer that dips below other rocks and is thus partially confined (section 2.8). Occasionally, wells tap only the confined section of such an aquifer, but in other cases wells also exist in the recharge area. In such systems carbon isotope studies are relevant for... 

Fig. 11.14 Apparent radiocarbon ages (see text) in shallow (dots) and deeper (circles) wells and age contours for the Watrak Shedi subbasin. The 2000-year values correspond to the recharge area (equivalent to an initial 14C of 80 pmc). The higher apparent ages indicate confinement of the aquifer in the lower half of the subbasin. A flow velocity of 6 m/year is calculated in the confined aquifer (see text). (From Borole et al., 1979.)...

Fig. 11.16 Study area of the Judean Mountains, central Israel. Limestone and dolomite (Cenomanian-Turonian) outcrops serve as recharge areas into a phreatic aquifer, confined on the flanks by younger chalk (Senonian). (From Mazor and Kroitoru, 1987.)...

Fig. 12.11 Sample locations in the Great Artesian Basin and concentrations of 36C1 (107 atoms/1) (data from Bentley et al., 1986a), and Cl, Ca, and S04 (mg/1) (data from Water Resources Commission of Queensland). Line DD marks a discontinuity between the 36C1, Ca, and S04 results from unconfined wells in the Great Dividing Range recharge area (located at the northeast corner) and the results from the buried confined J aquifer of the GAB.

The initial concentrations of atmospheric noble gases in recharge water can be predicted from the ambient annual temperature and altitude of the suggested recharge area. 

Recharge area Noble gases, t3 Sampling site ... 

Fig. 13.4 Definition of temperatures relevant to hydrological studies Ti = average temperature in the rainy season at recharge area T2 = average annual temperature at recharge area T3 = temperature at the base of the aerated zone, above the water table (deduced from the Ar, Kr, and Xe concentrations) T4 = maximum temperature reached at the deepest point of the water path T5 = observed spring or well temperature at the time of sampling T6 = ambient air temperature at the time of sampling. (From Herzberg and Mazor, 1979.)...

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