By Jay Lund, Thomas Harter, Rob Gailey, Rick Frank, and Graham Fogg
Groundwater problems are mostly invisible. However, as California has come to rely more on groundwater during the drought, land subsidence from groundwater drawdown and accumulating overdraft has become a visible concern in some areas.
Some of this subsidence has been dramatic. Almost 4 feet of subsidence occurred in some San Joaquin Valley areas over the past decade with current subsidence at a rate of one inch per month in the most severe cases. While most recent subsidence effects have occurred over a relatively short period, the rate of subsidence appears to be comparable to more widespread and dramatic land subsidence of about 30 feet that occurred in parts of the San Joaquin from 1925-1980 before large water projects imported surface water to reduced groundwater dependence.
Most land subsidence occurs the first time groundwater is drawn down to a new level. Compaction occurs as water flows from clays in the aquifer. This compaction is largely irreversible (Sneed et al. 2013). In contrast, sand and gravel strata can usually drain and be refilled with little subsidence or loss of storage capacity. (The land subsidence in the Sacramento-San Joaquin Delta is unrelated to groundwater pumping, but comes from continuing decomposition of peat soils due to agricultural development of former marshlands.)
Although land subsidence is often said to “destroy” an aquifer’s ability to store and transmit water, this is not actually the case. Aquifer compaction largely amounts to a one-time withdrawal of water from clays in the aquifer, and has little effect on overall ability of the aquifer to transmit water to wells. The clay compaction can reduce the ability of clays to transmit water to the aquifers (e.g., the sands) by vertical leakage, which could affect recharge to deeper aquifers.
This compaction also reduces the capacity of the aquifer system to store water by an amount equal to the amount of subsidence, but only in the clays (e.g., a foot of subsidence represents a foot of lost storage capacity in the aquifer system). If the amount of this subsidence is small compared to the overall thickness of the aquifer system, as is the case thus far in the Central Valley, the loss of system storage is small compared to the total water storage. On the other hand, if subsidence continues unabated, the capacity of the aquifer system to store water could be impaired more significantly.
The direct problems from recent land subsidence in the San Joaquin Valley due to groundwater pumping have been significant but not broadly catastrophic. Subsidence has not yet led to major problems for buildings, roads, railroads, or pipelines but some important problems have developed. Land subsidence from groundwater use has reduced slope and water conveyance capacity of some water supply canals and flood channels in the Central Valley, and could affect some well casings. Subsidence from unmanaged groundwater use is causing difficulties for delivering surface water, perhaps leading to more groundwater use. Groundwater-related land subsidence’s impacts in the Central Valley are greatest in reducing the capacities of regional flood channels. A major drought impact is likely to be decreased flood capacity and increased future flood damage and expenses. This has been a concern in both the Sacramento and San Joaquin valleys, but seems particularly acute for the Chowchilla bypass off the San Joaquin River for recent land subsidence.
The costs of addressing land subsidence can be sizable, but should be manageable. Most costs are likely from construction and land acquisition to restore flood and irrigation channel capacities, as well as additional flood damage, which may be relatively modest in the San Joaquin Valley. (Land subsidence costs in the Sacramento-San Joaquin Delta are far greater, causing far greater flood risk than subsidence elsewhere in California, but land subsidence in the Delta is caused by the oxidation and erosion of the Delta’s peat soils, not groundwater pumping.) Damage to homes and buildings has occasionally been reported, but the extent remains unclear.
The cause of most severe recent land subsidence impacts in California seems related to new irrigation development in some areas and drought-related groundwater pumping, both drawing groundwater down to new low levels. These impacts are focused on a few irrigation and flood control channels. In these cases, liability for damages would be desirable. The costs of damages to these facilities might be assessed proportionally to groundwater pumpers (or perhaps only new groundwater pumpers) in the underlying basin- a difficult legal question to answer.
Should these costs be assessed by difficult lawsuits under tort law, left to an arrangement under the basin plans required under the new Sustainable Groundwater Management Act, suffered by canal owners or flood victims (Ronald Coase lives!), or paid for by the public under some future water bond? And who should be responsible for pursuing these remedies? These seem to be the most relevant points of state policy to resolve while we avoid panic that the Earth is falling.
Borchers, J.W., V. Kretsinger Grabert, M. Carpenter, B. Dalgish, and D. Cannon (2014), Land Subsidence from Groundwater Use in California, Report for the California Water Foundation, Sacramento, CA. http://www.californiawaterfoundation.org/uploads/1397858208-SUBSIDENCEFULLREPORT_FINAL.pdf A nice inventory of land subsidence problems statewide related to groundwater pumping.
Galloway, D., D.R. Jones, and S.E. Ingebritsen (1999), Land Subsidence in the United States, Circular 1182, U.S. Geological Survey, Reston, VA, Chapter 6 on San Joaquin Valley. http://pubs.usgs.gov/circ/circ1182/pdf/06SanJoaquinValley.pdf
Sneed, M., J. Brandt, and M. Solt, 2013, Land subsidence along the Delta-Mendota Canal in the northern part of the San Joaquin Valley, California, 2003–10: U.S. Geological Survey Scientific Investigations Report 2013–5142, 87 p., http://dx.doi.org/10.3133/sir20135142 See also a nice write-up of this report and presentation at: http://mavensnotebook.com/2014/09/24/groundwater-pumping-and-subsidence-in-the-central-valley/