A new report from Ph.D. researcher Joshua W. Snarski and his team at the University of Connecticut reveals the details. Here, they dig further into groundwater dynamics occurring underneath temperate farmland fields. According to new modeling published in the journal Water Resources Research, replenishment of groundwater can occur from precipitation as old as one to two weeks. Crops are dependent for their sustenance on water that often comes from sources that are a thousand years older.
This study was conducted in the Horsebarn Hill agricultural catchment during the 2017 growing season. Researchers investigated how fast precipitation during this short period percolates through the vadose zone, or the layer of soil above the water table. This natural process can recharge the aquifer in as little as days to weeks. Such a finding is entirely counter to traditional, top-down hydrological models. Such models often operate under the assumption that recharge pathways are slow, with precipitation percolating for months to years through soil horizons and blending with legacy waters.
To figure out how old the water entering the groundwater system is, Snarski and his team tracked soil moisture levels. They further analyzed the isotopic composition of the groundwater for added perspective. They concentrated their efforts on pursuing oxygen isotopes—oxygen-18 and oxygen-16. Water molecules that contain the heavier isotope of oxygen, oxygen-18, evaporate and travel through the water cycle more slowly than lighter water molecules with oxygen-16. This distinction allows scientists to track how old the water is.
The field study consisted of three sampling sites across the Horsebarn Hill catchment that were characterized as representing different soil and groundwater layers. The researchers collected samples from the upper 20 cm layer, the underlying 20-40 cm layer above the water table, and the groundwater layer itself. By analyzing seasonal average ages of water stored in soil during the growing season, they were able to draw significant conclusions about groundwater dynamics.
Recent young water from recent precipitation is highly effective at recharging aquifers. Mined or legacy water sources are integral to crop irrigation. This distinction illustrates the nuanced interactions between surface precipitation and groundwater systems. Comprehension of these processes is necessary to better inform agricultural production practices and water resource allocation planning.