Land Surface Subsidence In Southwestern Louisiana Caused by Seasonal Groundwater Withdrawal

Vermillion, Acadia, and Jefferson Davis Parishes in Southwest Louisiana withdrew on average more than 450 million gallons of groundwater per day in 2000. The primary aquifer in this region is the Chicot Aquifer. Water enters the groundwater system in Southwest Louisiana as recharge in an outcrop area north of Acadia and Jefferson Davis Parishes. Water moves downward and southward by topography-driven recharge. Most of the groundwater withdrawn in Vermillion, Acadia, and Jefferson Davis Parishes is used for irrigation of agricultural fields and thus the use is seasonal. During the growing season, water usage is much higher than average and water levels in wells can drop by more than 12 meters. After the growing season, water withdrawals decline and water levels in wells largely rebound to their pre-growing season levels. In addition, there has been a slow long-term decline in water levels in the Chicot aquifer due to over pumping both for irrigation and industrial use since 1940. Most surface subsidence due to groundwater withdrawal is believed to be the result of clay compaction rather than compaction of the aquifer itself. As water pressure levels decrease in major aquifers in response to pumping, water also drains from interbedded clay confining layers. The weight of overlying sediments causes clays to compact. The clay compaction effect on surface subsidence due to long-term groundwater level declines has been well documented in Houston, Texas; Las Vegas, Nevada; Sacramento Valley, California; and Venice, Italy. While water levels in aquifers due to seasonal withdrawals may recover, water levels in adjacent confining layers do not. The lower permeability of clays causes them to progressively drain with each seasonal cycle of groundwater withdrawal. Thus, permanent surface subsidence is possible by this mechanism.