Principal Investigator Charles Harvey
Project Website http://web.mit.edu/harvey-lab/hydrogeology.html
We collect: (1) Water levels in the wells, ponds and rivers, (2) pumping rates from irrigation wells, (3) hydrogeologic mesures (pump tests, well logs), and (3) meteorological data. The combined data show that:
(*) The upper sand aquifer, from which most wells pump, has little storage because- a thick (2-4m) surface layer of silty-clay confines it.
(*) During the dry season (December to April) net atmospheric input is negative -- evapotranspiration is greater than rainfall.\t
(*) During flooding (June to November) groundwater flow is negligible because Water levels in the aquifer and ponds equilibrate with the overlying flood waters and there Is no irrigation pumping.
(*) irrigation pumping has more than doubled the flow of water through- the -aquifer. Pond construction has also greatly increased the flow through the aquifer. \t-
Three important hydrogeologic characteristics:
(1) The upper aquifer acts primarily as a conduit for flow from ponds and rice fields to irrigation wells and rivers. The volume of water stored in-the aquifer changes little seasonally. Most flow into the aquifer occurs during the dry season through low permeability pond sedimentsiandrice fields Because net atmospheric input is negative (evaporation > rain), recharge comes only from these locations where inflow is available. Much of this flow occurs even as aquifer heads are-deciinlng Most flow out of the aquifer is through irrigation wells and into the river. The quantity of water that enters the aquifer when hydraulic heads rise during the onset of the rainy season is relatively small - it takes little water to raise the head in the aquifer because aquifer storage is small.
(2) Arsenic concentrations are not In a steady state. Irrigation pumping and pond construction has increased the flow of water through the aquifer. This increased water flow will dilute arsenic concentrations unless more arsenic is mobilized into the groundwater. Arsenic concentrations cannot remain unchanged unless the rate of arsenic mobilization has Increased by the proportion necessary to match the increased flow of water. In fact, for arsenic concentrations torémain constant, the rate of arsenic mobilization would have to change spatially to match the new complex spatial pattern of flow. The rate of mobilization has likely increased, but there is no reason to believe the pattern and magnitude of increased mobilization matches the pattern of Increased flow.
(3) Pumping from the deep aquifer will not de-water this aquifer. The current rate of flow Into this aquifer is irrelevant for calculating the sustainable yield of the aquifer because this aquifer draws more water from above when the head is lowered. The resource of concern is not the water, but the hydraulic head in the aquifer -- if the head is greatly lowered, lifting water to the surface will become more expensive even though the deep aquifer remains saturated year-round.