By Rich Pauloo and Graham Fogg

Lower groundwater levels can prevent drainage of water and salts from a basin and increase aquifer salinity that eventually renders the groundwater unsuitable for use as drinking water or irrigation without expensive desalination. Pauloo et al. (2021)  demonstrate this process for the Tulare Lake Basin (TLB) of California’s Central Valley. Even if groundwater pumping does not cause overdraft, it can cause hydrologic basin closure leading to progressive salinization that will not cease until the basin is opened by allowing natural or engineered exits for groundwater and dissolved salt. The process, “Anthropogenic Basin Closure and Groundwater Salinization (ABCSAL)”, is driven by human water management. 

Salts do not accumulate in aquifers when outlets exist to discharge groundwater and its salt load from the basin (Figure 1A). Salt load (TDS) is inherent to groundwater due to natural, ubiquitous subsurface rock-water weathering reactions. Many closed basins without salt drainage exist globally, including Death Valley, Salar de Uyuni (Bolivia), and saline lakes like the Great Salt Lake (USA) and the Dead Sea (Middle east). In all of these basins, the dominant exit for water, including groundwater, is evaporation that leaves salts behind. Changes in groundwater management, such as more pumping, recharge, or evapotranspiration can change the groundwater quality sustainability by merely closing the basin. This study investigated the time scales and ultimate magnitudes for groundwater basin salinization that can be expected in the TLB as it has shifted to a closed basin due to pumping and irrigation. 

Prior to groundwater use for irrigation in California, the TLB was drained by baseflow to surface water, lateral subsurface flow, and episodic spill from Tulare Lake into the San Joaquin Valley to the north (Figure 1B). A basin can transition from open or “drained”, to closed or “undrained”, when groundwater pumping lowers groundwater levels so saturated groundwater becomes disconnected from streams, and the direction of lateral groundwater flow reverses, causing the basin to drain adjacent areas (Figure 1B). Thus, the basin is “closed” in the sense that groundwater can no longer drain salts out of the basin. Closed basins naturally salinate over time.

The model results for the TLB show that salinization proceeds from the top of the aquifer down, as recharge water drives evapoconcentrated water at the land surface into shallow and then deeper aquifers over decadal to century long timescales (Figure 3). Impacts occur in shallow aquifers (around 100 feet deep) within decades, and in deep aquifers (greater than 500 feet) within two to three centuries. Results agree with measured TDS changes (Hanson et al., 2018; Pauloo et al., 2021) in shallow aquifers from historic to modern times in the TLB. The causes of basin closure are groundwater pumping and evapotranspiration (ET) from irrigated crops. Over the last century in the TLB, exits for groundwater have shifted from baseflow and lateral subsurface flow to ET, which now accounts for nearly all groundwater discharge and accommodates no salt discharge (Figure 2). 

Groundwater basins can become closed or “undrained” due to moderate amounts of pumping, even without chronic declines in groundwater storage or overdraft. If the dominant land use in these basins is irrigation, then salinization from ABCSAL is likely already underway and if unchecked, the groundwater will eventually become unusable without expensive desalination. Pauloo et al. (2021) show the timescale of this process in the TLB is similar to groundwater basin exhaustion from overdraft. This raises the interesting question: “How could ABCSAL be avoided?” The answer is simple — the hydrologic basin would have to be opened (become “drained”) by managing it in a way that allows exits for groundwater and its entrained salts by baseflow to streams and wetlands, lateral flow to adjacent basins, or regional agricultural drains. This would require careful groundwater monitoring and management that maintains water table elevations such that the basin is sufficiently “full” to drain groundwater and salt to the surface and eventually to destinations beyond the basin. 

In groundwater basins undergoing salinization and significant overdraft, like the Tulare Lake Basin, it may seem far-fetched to suggest the hydrologic balance of these systems can ever be reversed sufficiently to open them. In basins not yet in advanced stages of overdraft and ABCSAL, however, it would be prudent to develop groundwater management that moderates pumping while maximizing recharge to maintain hydrologically open, “drained” conditions. A necessary ingredient in this water management would likely emphasize subsurface storage of water much more than in the past, possibly prioritizing subsurface water storage over the more common and familiar surface storage. 

The Central Valley has at least three times the subsurface water storage “space” than California’s entire surface reservoir storage capacity, highlighting opportunities to better use subsurface storage. Basin salinization challenges long term groundwater quality sustainability under the Sustainable Groundwater Management Act (SGMA). Solutions to slow or reverse salinization should emphasize managed aquifer recharge to increase groundwater storage, improve water quality, reduce pumping costs, and secure clean irrigation and drinking water. 

Dr. Rich Pauloo (www.richpauloo.com) is a scientist at Larry Walker Associates and a co-founder of the Water Data Lab (www.waterdatalab.com).  Dr. Graham Fogg is a Professor Emeritus of Hydrogeology and Hydrogeologist in the Agricultural Experiment Station at the University of California Davis.

Further reading

Hansen, J. A., Jurgens, B. C., & Fram, M. S. (2018). Quantifying anthropogenic contributions to century-scale groundwater salinity changes, San Joaquin Valley, California, USA. Science of the total environment, 642, 125-136.

Pauloo, R. A., Fogg, G. E., Guo, Z., & Harter, T. (2021). Anthropogenic basin closure and groundwater salinization (ABCSAL). Journal of Hydrology, 593, 125787.