Water Resources of Iberville Parish, Louisiana

From a U.S. Geological Survey fact sheet of 2021, prepared for water managers and residents.

In 2014 Iberville Parish, in southeastern Louisiana, withdrew about 589.87 million gallons of water a day (Mgal/d): 30.86 Mgal/d of groundwater and 559.01 Mgal/d of surface water.

Water-use data gathered every 5 years from 1960 to 2010, and in 2014, show withdrawals peaked in 1980 at 1,429.78 Mgal/d.

Water withdrawals in Iberville Parish, 1960–2014. Credit: U.S. Geological Survey.

Groundwater

Iberville Parish lies in the Lower Mississippi alluvial valley, the transition between the Chicot, Evangeline and Jasper aquifer systems to the west and their "equivalent" systems to the east. Here those deep sediments mostly hold salt water.

• The Mississippi River alluvial aquifer is the main source of fresh groundwater — sediments filling a valley cut into the older Chicot equivalent aquifer system. The Atchafalaya aquifer, gravel filling channels in the old Mississippi floodplain, may reach the far west of the parish and is counted with it.
• The Chicot equivalent aquifer system is directly connected to the alluvial aquifer; beneath it lie the Evangeline equivalent and then the Jasper equivalent aquifer systems. They hold little available fresh water and are pumped far less.
• Fresh groundwater (chloride of 250 mg/L or less) reaches 145 feet to more than 500 feet below the NGVD 29 datum in the east of the parish. Elsewhere it is absent from all the major aquifers except in the far north, where it extends into the Evangeline and upper Jasper equivalent systems — an abrupt change possibly linked to the Baton Rouge–Tepetate fault zone. With so little groundwater fit for public supply, the City of Plaquemine gets its water from wells in West Baton Rouge Parish.

The study area, Iberville Parish, Louisiana. Base modified from U.S. Geological Survey digital data. Credit: U.S. Geological Survey.

Mississippi River alluvial aquifer

Its deposits grade from silt and clay at the surface to coarse sand and gravel at the base, which forms the aquifer; the upper 75 to 100 feet of silt and clay confine it. In places the Mississippi cuts through that layer and connects to the aquifer, and water levels in the aquifer follow the river's stage, swinging most in wells near it.

• Levels. Measurements at 18 wells from 1990 to 1992 ranged from 7.45 feet below to 25.54 feet above NGVD 29. Well Ib-106 shows the river's influence from 1959 to 2016 but no long-term trend.
• Wells. 403 active wells in 2017 (including 5 in the Atchafalaya aquifer): 217 domestic, 79 irrigation, 70 industrial, 34 public-supply and 3 for power generation; 30 to 733 feet deep; yields of 4 to 7,198 gallons a minute.
• Use. About 26.72 Mgal/d in 2014: 21.41 for industry, 1.68 general irrigation, 1.50 aquaculture, 0.97 power generation, 0.92 public supply, 0.15 rural domestic and 0.09 livestock.

The deeper systems

• Chicot equivalent — alternating beds of sand and clay laid down before the modern valley was cut; equivalent to Baton Rouge's "400-foot" and "600-foot" sands and New Orleans' Gonzales–New Orleans aquifer. Its upper sands act with the alluvial aquifer as one unit in places. 3.68 Mgal/d were withdrawn for industry in 2014, from one active well.
• Evangeline and Jasper equivalents — fresh only in the north. In the southeast their tops lie about 1,000 and 2,500 feet below NGVD 29. Water levels have fallen in several northern wells: well Ib-9 in the Evangeline equivalent ("1,200-foot" sand) dropped about 57 feet in 1951–86, and well Ib-29 in the Jasper equivalent ("2,000-foot" sand) 84 feet. In 2017 there were one public-supply well in the Evangeline equivalent and six in the Jasper equivalent.

Water levels in wells Ib-9 (Evangeline equivalent), Ib-29 (Jasper equivalent) and Ib-106 (Mississippi River alluvial aquifer). Credit: U.S. Geological Survey.

Groundwater quality

Samples from 142 wells in the alluvial aquifer (1932–85) and 36 in the Jasper equivalent system (1942–90, mostly in the north and neighbouring parishes):

Surface water

Five subbasins drain the parish: the Atchafalaya and Lower Grand, which cover most of it; the Lower Mississippi–Baton Rouge, a narrow strip along the river; and the Amite and West Central Louisiana Coastal, draining small areas in the east. Surface-water withdrawals in 2014 were 427.71 Mgal/d for industry, 123.58 for power generation, 6.00 for aquaculture, 1.12 for general irrigation, 0.58 for public supply and 0.03 for livestock.

• Atchafalaya subbasin — the west of the parish, east to Bayou Maringouin. The Atchafalaya River runs along or forms the western border; with its swamps it supports fishing, hunting and other activities, and its floodplain carries Mississippi floodwater. An average of 25 percent of the Mississippi's flow (1984–2007) is diverted into it through the Old River Control Structure, and spring floods sustain its wetlands. At Simmesport its daily average flow in 1963–2016 was 225,520 cubic feet per second, highest in May (342,173) and lowest in September (108,526). Bristow Bayou and Bayou Sorrel flow south-southwest towards it.
• Lower Grand subbasin — most of the parish east of Bayou Maringouin, draining south through Bayou Grosse Tete, Bayou Tigre and Bayou Goula into the Lower Grand River, which averaged 2,103 cubic feet per second at Bayou Sorrel in 2013 and supplied 0.58 Mgal/d for public supply in 2014. It includes the Gulf Intracoastal Waterway, which meets the Mississippi at the Port Allen Lock.
• Lower Mississippi–Baton Rouge subbasin — small in area, but the Mississippi is a vital source: it averaged about 460,000 cubic feet per second near Tarbert and Red River Landings in 1928–76, and 551.28 Mgal/d were withdrawn from it in the parish in 2014, 78 percent for industry and 22 percent for power.
• Amite (Bayou Paul and Bayou Braud, flowing east) and West Central Louisiana Coastal (Bayou Sigur and Grand Bayou, flowing south).

Surface-water quality — samples from the Mississippi at Plaquemine (1973–93), the Lower Grand River at Bayou Sorrel (1956–86) and the Atchafalaya at Krotz Springs (1952–72) were generally within secondary standards for pH, chloride, sulfate and dissolved solids, and hard at all three. At the Mississippi and Lower Grand sites, iron and manganese were generally below their standards and median dissolved oxygen was above 5 mg/L, the minimum for diverse warm-water life. Median colour was above the standard of 15 platinum-cobalt units at the Lower Grand River and at it for the Mississippi and Atchafalaya.

Sources

• Lindaman, M.A., and White, V.E., 2021, Water resources of Iberville Parish, Louisiana: U.S. Geological Survey Fact Sheet 2021–3014. https://doi.org/10.3133/fs20213014
• Water-use figures are from Collier (2018) and Sargent (2011); well records from the Louisiana Department of Natural Resources (2017).
• Corrected: the source calls industry's share one of the water "withdrawn in 2016"; its figures are for 2014.
• The map and figures are taken from the fact sheet's PDF.
• Rewritten in hubnx's own words.