Water Resources of St. Landry Parish, Louisiana

St. Landry Parish: the approximate base of fresh groundwater, aquifer limits, streams and monitoring sites. USGS.

Knowing how much water St. Landry Parish, Louisiana, has, how it's used and how good it is helps manage its supply. This USGS fact sheet, prepared with the Louisiana Department of Transportation and Development, draws on earlier reports and the USGS National Water Information System.

Water use

In 2014 the parish withdrew about 116.75 million gallons per day (Mgal/d): 98.13 from groundwater and 18.62 from surface water. Agriculture — rice irrigation, general irrigation, aquaculture and livestock — took about 90 percent (105.31 Mgal/d); public supply about 8 percent (9.77), industry about 1 percent (1.03) and rural domestic about 1 percent (0.65). Withdrawals, recorded every 5 years since 1960, peaked in 1965 at 194.57 Mgal/d because of a one-time 144.00 Mgal/d surface-water withdrawal for power generation.

Water withdrawals, 1960–2014. USGS.

Groundwater

Three freshwater aquifers serve the parish: the Atchafalaya aquifer, the Chicot aquifer system and the Evangeline aquifer. A surficial confining unit of clay, silt and sand, 40 to 160 feet thick, covers the parish; it slows leakage downward — and makes the land ideal for rice.

• Base of fresh water (chloride 250 mg/L or less): as deep as 2,000 ft below the NGVD 29 datum along the eastern edge, where fresh water reaches the Evangeline; elsewhere from 150 ft below in the north and center to about 700 ft in the south.

An idealized north-to-south section through the parish's aquifers (vertical scale exaggerated). USGS.

• Water levels: Atchafalaya levels follow the Atchafalaya River's stage and dip seasonally with rice and aquaculture pumping; well SL-208 shows short swings and a slight decline. In the Chicot, water flows east to west toward heavy pumping in Evangeline and Acadia parishes; well SL-179 fell about 20 ft from 1957 to 2015, and falling levels have let saltwater reach some irrigation wells since the late 1990s. Evangeline well SL-220 fell about 34 ft from 1967 to 2006, possibly from pumping in Evangeline Parish.
• Yields: individual Evangeline sands give 100–1,000 gal/min, versus 500–2,500 for the combined Chicot and Atchafalaya.

Water levels in wells SL-208 (Atchafalaya), SL-179 (Chicot) and SL-220 (Evangeline). USGS.

Groundwater quality

Samples from 274 Chicot wells (1940–2001) and 25 Atchafalaya wells (1942–80) were generally within EPA's secondary (aesthetic) limits for pH, color, chloride and sulfate. The water is hard — median hardness 290 mg/L (very hard) in the Atchafalaya and 170 mg/L (hard) in the Chicot — and most samples exceeded the limits for manganese (50 µg/L) and iron (300 µg/L).

Surface water

Four subbasins drain the parish, generally south. In 2014, 18.62 Mgal/d of surface water was withdrawn — aquaculture 7.30, rice 7.33, general irrigation 3.96, livestock 0.03.

• Bayou Teche (largest, central): fed by Bayou Boeuf, Bayou Cocodrie and Bayou Wauksha. At Arnaudville it averaged about 1,047 ft³/s (1983–2004), highest in February (1,164) and lowest in October (944). In 2014, 4.83 Mgal/d each came from Bayou Teche and Bayou Boeuf, split between rice and aquaculture.
• Atchafalaya (east): the Atchafalaya River forms the parish's eastern border; its swamps support fishing and hunting, its floodplain carries Mississippi River floodwater, and an average 25 percent of the Mississippi's flow was diverted into it through the Old River Control Structure (1984–2007). At Simmesport it averaged 225,520 ft³/s (1963–2016), highest in May (342,173) and lowest in September (108,526). Bayou Cortableau and Bayou Rouge drain into it.
• Vermilion and Mermentau Headwaters (southwest): the Vermilion River begins near the parish's southern border where Bayou Bourbeux, Coulee Rouge, Coulee de Marks and Bayou Sandy meet; Bayou Mallet and Bayou Plaquemine Brule drain the far west.

Quality: samples from the Atchafalaya River at Krotz Springs (1952–72) generally met the limits for pH, chloride, sulfate and dissolved solids, with median hardness of 130 mg/L (moderately hard).

Sources

Based on Water Resources of St. Landry Parish, Louisiana, by Maxwell A. Lindaman and Vincent E. White, USGS Fact Sheet 2021–3026, prepared with the Louisiana Department of Transportation and Development, U.S. Geological Survey (DOI), drawing on Collier (2018), Sargent (2004, 2011), Milner and Fisher (2009), Whitfield (1975), Nyman (1984) and USGS data; a work of the United States government in the public domain. The figures, the use table and the Atchafalaya River's monthly extremes come from the fact sheet's PDF.