Groundwater cycle

Fig. 3. Example of similarity between atmospheric C02 and groundwater C02 annual cycles. The groundwater cycle lags the atmosphere cycle by 6 months.

T3—the temperature at the base of the aerated zone, just above the water table. This temperature is deducible from the noble gas concentration. T4—the highest temperature reached during the groundwater cycle, presumably obtained at the deepest point of circulation. 

The efficiency of the weathering of rocks in using carbonic acid produced in the carbon cycle is affected by various hydrologic, environmental, and cultural controls. The fact that the principal anion in fresh surface water worldwide almost always is bicarbonate attests to the overriding importance of this process. Exceptions are systems in which evaporite minerals are available for dissolution by groundwater or where human activities are major sources of sulfate or chloride inflow. 

The oceans hold about 97% of the earth s water. More than 2% of the total water and over 75% of the freshwater of the world is locked up as ice ia the polar caps. Of the remaining 1% of total water that is both Hquid and fresh, some is groundwater at depths of > 300 m and therefore impractical to obtain, and only the very small difference, possibly 0.06% of the total water of this planet, is available for human use as it cycles from sea to atmosphere to land to sea. Only recently have humans been able to regulate that cycle to their advantage, and even now (ca 1997), only infinitesimally, ia some few isolated places. 

Many hydrologic reservoirs can be further subdivided into smaller reservoirs, each with a characteristic turnover time. For example, water resides in the Pacific Ocean longer than in the Atlantic, and the oceans surface waters cycle much more quickly than the deep ocean. Similarly, groundwater near the surface is much more active than deep reservoirs, which may cycle over thousands or millions of years, and water frozen in the soil as permafrost. Typical range in turnover times for hydrospheric reservoirs on a hillslope scale (10-10 m) are shown in Table 6-4 (estimates from Falkenmark and Chapman, 1989). Depths are estimated as typical volume averaged over the watershed area. 

The excess of evaporation from the oceans is made up for by runoff from the land. Although this flux is much smaller than precipitation and ET, it is a major link in many cycles and is of particular importance to humans in terms of water supply. Runoff can be broadly categorized into subsurface, or groundwater, flow and surface flow, consisting of overland runoff and river discharge. 

Krabbenhoft DP, Babiarz CL. 1992. The role of groundwater transport in aquatic mercury cycling. Water Resources Res 28 3119-3128. 

Understanding the behavior of radionuclides in estuaries, as the dynamic interface between the continental hydrochemical systems and the ocean basins, requires consideration of broader chemical cycling in the hydrosphere. In this volume, the behavior of U- and Th-series isotopes in rivers is discussed by Chabaux et al. (2003), that in groundwaters by Porcelli and Swarzenski (2003), and that in oceans by Cochran and Masque (2003). General background information is provided by Bourdon et al. (2003). 

In nature, the groundwater is a part of the hydrological cycle. Hence, groundwater is naturally recharged and drained. Sometimes the draining is shown up as springs, but more common it flows out to a lake or a river as shown in Figure 34. 

Soil compartment chemical fate modeling has been traditionally performed for three distinct subcompartments the land surface (or watershed) the unsaturated soil (or soil) zone and the saturated (or groundwater) zone of a region. In general, the mathematical simulation is structured around two major cycles the hydrologic cycle and the pollutant cycle, each cycle being associated with a number of physicochemical processes. Watershed models account for a third cycle sedimentation. 

The hydrologic cycle, or moisture cycle — that may encompass the processes of rain infiltration in the soil, exfiltration from the soil to the air, surface runoff, evaporation, moisture behavior, groundwater recharge and capillary rise from the groundwater. All these processes are interconnected and are frequently referred to as the hydrologic cycle components. 

From the hydrologic cycle temporal resolution of soil moisture surface, runoff, and groundwater recharge components, by inputting to the model the net infiltration rate into the soil column and... 

If 0.24 Pg C/a represents riverine DIC delivered to oceans (Meybeck 1993) and if the flux of carbon from rivers/lakes to the atmosphere is 20% (Kling et al. 1991) of the total (i.e., 0.12 Pg C/a), then 0.23 Pg C/a remains in inland lakes and rivers, and in slowly cycled groundwater. Cole et al. (2007) estimated that about 0.2 Pg C/a is buried in inland water sediments. Groundwater may have a greater carbon storage capacity due to its large volume and greater load of carbon than rivers (Kempe 1984). 

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