Theoretical head decay in closed basin aquifers: an insight into fossil groundwater and recharge events in the Andes of northern Chile

Abstract

Many small to moderate-sized, closed aquifer basins in northern Chile show a groundwater gradient indicating flow despite an apparent lack of recharge under current climatic conditions. An examination of the mechanisms that might produce such flow suggests that the only realistic possibility is head decay from an earlier higher water level caused by ancient recharge. Model studies suggest that for small basins, such recharge must have taken place within the last 1000 years and possibly within the last few hundred years, calling into question whether such resources should be labelled fossil. Recent palaeoclimatic studies of northern Chile show several pluvial periods throughout the Late Pleistocene and Holocene, inaddition to long return period storm events, which continue to the present and which might have led to a series of pulsed flushing events in aquifers. Hence, it is incorrect to label groundwater resources as either fossil or actively recharged, as there is, in reality, a continuum between the two, and all such aquifers require site-specific evaluation and new, more flexible guidelines for development.

Timing of wet episodes in Atacama Desert over the last 15 ka. The Groundwater Discharge Deposits (GWD) from Domeyko Range at 25°S.

“…Studying deposits formed during these wet periods is a first step towards assessing water runoff and replenishment of groundwater reserves (Houston and Hart, 2004;Herrera and Custodio, 2014). These pluvial events were also vital to human subsistence following the initial occupation of the region between 11.5 and 13 ka ago (Moreno et al, 2009;Santoro et al, 2011;Gay o et al, 2015) as well as the last two centuries when water needs for mining, agriculture and human consumption have greatly increased.…”

Recharge to groundwater in the Turi Basin, northern Chile: An evaluation based on tritium and chloride mass balance techniques

“…On the contrary, Stimson et al (1996) show that 3 H can be used to identify recharge and flow paths in phreatic Andean aquifers. Furthermore, Houston and Hart (2004) have shown that many small Andean basins contain groundwater that must receive intermittent recharge in order to maintain gradients and playa evaporation. The chloride (Cl) mass balance technique (Allison and Hughes, 1978;Dettinger, 1989) has been shown to provide an additional approach to the estimation of recharge that has not previously been applied in the Andean Cordillera.…”

Composition of groundwaters associated with porphyry-Cu deposits, Atacama Desert, Chile: Elemental and isotopic constraints on water sources and water–rock reactions

“…Their compositions (À11& to À9& for d 18 O VSMOW and À90& to À80& for d 2 H VSMOW ) are consistent with recharge in the Andes at elevations around 3500-4000 m (Aravena et al, 1999). Rissmann (2003) suggested that dilute groundwaters in the Monturaqui Basin, proximal to the Andes, may have been recharged at elevations lower than 3500 m during wetter periods in the Pleistocene (54.4-15.3 ka) and short periods in the Holocene (11.4-10.2 and 6.2-3.5 ka) (Bobst et al, 2001) (Houston and Hart, 2004).…”

“…The hyperarid climate has reduced the rate of surface erosion to $1 cm/ka since that time (Alpers and Brimhall, 1988). There have been, however, more pluvial periods during this time (Bobst et al, 2001;Houston and Hart, 2004).…”