Water Resources of Iberia Parish, Louisiana

Knowing how much water Iberia Parish, Louisiana, has, how it is used and how good it is matters for managing it. This U.S. Geological Survey fact sheet, prepared with the Louisiana Department of Transportation and Development, draws on earlier reports and the USGS National Water Information System to give water managers, residents and others that picture.

How much water is used

In 2010 the parish withdrew about 31.24 million gallons a day (Mgal/d): 23.13 from groundwater and 8.11 from surface water. Public supply and industry each took about 32 percent. Records kept every five years since 1960 show withdrawals peaking at about 58.57 Mgal/d in 1975.

Groundwater and surface-water withdrawals in Iberia Parish, 1960–2010. USGS.

Groundwater

The main sources of fresh groundwater — water with no more than 250 milligrams per litre (mg/L) of chloride — are the Atchafalaya aquifer and the Chicot aquifer system. The base of fresh water lies from less than 200 feet below the National Geodetic Vertical Datum of 1929 (NGVD 29) in the east to more than 800 feet below in the northwest; at the parish's eastern end there is little or none.

Aquifers here are recharged by rain, by water moving sideways from other aquifers, and seasonally by rivers. The Atchafalaya River is the main source for the Atchafalaya aquifer; the Chicot system is recharged largely by leakage down through surface clays, especially in coastal wetlands. The Atchafalaya aquifer loses water to wells, to the river when it is low, and sideways into the Chicot system, drawn by pumping from it. The Chicot system loses water to wells and to flow west and north, drawn by heavy pumping in southwestern Louisiana. North of the parish the two are closely connected; south of St. Martinville they are generally poorly connected.

Iberia Parish: depth to the base of fresh groundwater, subbasins, and the well and river sites described here. USGS.

The Atchafalaya aquifer

In the parish, the aquifer holds fresh water from near Bayou Teche east to a northwest–southeast line about 8 miles west of the Assumption Parish line; beyond it holds only saltwater. It is the sand and gravel of Atchafalaya and Mississippi River deposits, under a thick layer of clay. The base of these deposits lies less than 100 feet below NGVD 29 near Bayou Teche and about 375 feet below under Lake Fausse Pointe — about 225 to 275 feet below in the north, and 250 to 275 feet at the eastern end.

In 2015 the state listed 275 active wells in it — 244 domestic, 19 irrigation and 12 public supply — about 70 to 262 feet deep, yielding 5 to 500 gallons a minute. In 2010 it supplied about 3.51 Mgal/d, almost all for aquaculture (3.33).

The Chicot aquifer system

The Chicot is a regional aquifer system under southwestern Louisiana. In Iberia Parish it has four parts:

• a surface confining layer of clay, from less than 40 feet thick in the northwest corner to more than 320 feet near the coast on the Vermilion Parish border;
• shallow sands — scattered streaks, lenses and layers within that clay, which can supply homes;
• the upper sand, the main aquifer: mostly coarse sand grading to gravel at the base of each bed, with its top 200 to more than 400 feet below NGVD 29 and its base from less than 600 feet in the north to more than 800 feet in the south. It generally holds fresh water west of the Atchafalaya River, but mostly over saltwater, except in a small area in the northwest corner;
• the lower sand, which generally holds only saltwater here.

Water levels in the upper sand rise and fall with seasonal demand, as at well I-93, east of New Iberia, where they fell about 2 feet from 1990 to 2015. In 2003, levels ranged from above NGVD 29 east of that well to 0–20 feet below along the Vermilion Parish border — lower in the west mainly because of pumping in parishes to the west and north.

In 2015 there were 1,862 active wells in the Chicot system — 1,553 domestic, 162 public supply, 75 irrigation and 72 industrial — about 60 to 856 feet deep, yielding 3 to 3,800 gallons a minute. In 2010 it supplied about 19.6 Mgal/d: 10.07 for public supply, 4.40 for industry, 3.33 for aquaculture, 1.04 for rural homes, 0.61 for general irrigation, 0.12 for rice and 0.05 for livestock.

Groundwater quality

Water from the main aquifers is of similar quality. Samples from the Chicot upper sand collected from 1940 to 2009 were very hard; iron and manganese generally exceeded the Environmental Protection Agency's secondary maximum contaminant levels (SMCLs), non-enforceable guidelines on cosmetic, taste and technical effects; median pH was within the guidelines, and dissolved solids generally were. Samples from the Atchafalaya aquifer west of the Atchafalaya River had iron of 1,000 to more than 5,000 micrograms per litre, hardness generally above 500 mg/L as calcium carbonate, pH of about 6.5–7.5, and specific conductance of 500 to 1,000 microsiemens per centimetre at 25 °C.

Surface water

Most of the parish's surface water lies in the Atchafalaya–Vermilion basin, divided here into the Vermilion, Bayou Teche and Atchafalaya subbasins, with a little at the eastern end in the West Central Louisiana Coastal and Lower Grand subbasins. In 2010 about 8.11 Mgal/d of surface water was withdrawn: 5.45 for industry, 1.67 for aquaculture, 0.98 for rice and 0.01 for livestock.

• Vermilion subbasin, west of Bayou Teche, drains south toward Vermilion and West Cote Blanche Bays. It holds the Gulf Intracoastal Waterway, inland and along the shore of Vermilion Bay, as well as Lake Peigneur, Spanish Lake, Bayous Petite Anse, Carlin and Patout, and many streams and canals. In 2010 industry drew 5.03 Mgal/d from the Intracoastal Waterway and 0.38 from Bayou Patout, and 1.81 Mgal/d came from other waters, mostly for aquaculture and rice.
• Bayou Teche subbasin, in the middle of the parish, reaches east to include Lake Fausse Pointe, a large, shallow lake shared with St. Mary Parish and separated from the Atchafalaya floodway by a levee: at about 2 feet above NGVD 29 it covers about 24 square miles, with an average depth of about 3 feet. Bayou Teche at Adeline Bridge near Jeanerette averaged 416 cubic feet per second in 1996–2015; the flow there is tidal, sometimes reversing or stopping. In 2010, 0.87 Mgal/d was taken from the bayou for aquaculture and rice.
• Atchafalaya subbasin, between two north–south flood levees east of Lake Fausse Pointe and near the eastern end of the parish. The Atchafalaya River is an important shipping channel, and its floodplain carries Mississippi River floodwaters, supports commerce and recreation, and is home to a great variety of plants and wildlife. Most of the water leaves through the Lower Atchafalaya River or the Wax Lake Outlet in St. Mary Parish, which in 1995–2014 averaged 125,700 cubic feet per second at Morgan City and 96,310 at Calumet.

Surface-water quality

Samples from the Lower Atchafalaya River at Morgan City (2006–9) and Bayou Teche at Keystone Lock near St. Martinville (1965–98) were generally within the guidelines for pH, chloride and sulfate. The Atchafalaya was within the iron guideline, but Bayou Teche exceeded it in more than 20 percent of samples. Median hardness was hard in the Atchafalaya and soft in Bayou Teche. Median dissolved oxygen was above 5 mg/L at both, the level considered the minimum for a diverse community of warm-water life, including sport fish.

Sources

• U.S. Geological Survey, in cooperation with the Louisiana Department of Transportation and Development, Water Resources of Iberia Parish, Louisiana, Fact Sheet 2016–3100, February 2017. https://doi.org/10.3133/fs20163100
• USGS Lower Mississippi-Gulf Water Science Center: http://la.water.usgs.gov