Water Resources of Pointe Coupee Parish, Louisiana

Knowing how much water Pointe Coupee Parish, between the Atchafalaya and Mississippi rivers in south-central Louisiana, has, how it is used and how good it is helps water managers and residents look after it. This U.S. Geological Survey fact sheet, prepared with the Louisiana Department of Transportation and Development, sums it up.

How the water is used

In 2014 the parish withdrew about 364 million gallons a day (Mgal/d): about 39.87 from groundwater and 323.72 from surface water.

Agriculture — aquaculture, irrigation and livestock — took 8 percent (29.29 Mgal/d). Figures are rounded, so they may not add up exactly.

Water withdrawals, 1960–2014, measured every five years to 2010 and again in 2014. They peaked in 2014. The jump in surface-water use between 1980 and 1985 came from an extra 262 Mgal/d for power generation.

The aquifers

Pointe Coupee Parish. Shading shows the depth of the base of fresh groundwater below NGVD 29; A–A′ is the section below. From the USGS fact sheet.

The main freshwater aquifers are the alluvial aquifer and the Chicot, Evangeline and Jasper equivalent aquifer systems. Across most of the parish, fresh groundwater — 250 milligrams per litre (mg/L) of chloride or less — extends down into the Jasper system, from about 300 feet below sea level (NGVD 29) at the northern tip to more than 2,500 feet below it in the south. In the west-central and southwest, fresh water ends in the Evangeline system, between less than 700 and more than 2,000 feet below NGVD 29, with some salty layers in between.

An idealised north-to-south section through the parish. The sands are named for their depth in the Baton Rouge area.

Alluvial aquifer and Chicot equivalent system

The alluvial aquifer is the shallowest. Called the Mississippi River alluvial and Atchafalaya River alluvial aquifers in different places, it is one connected aquifer across the whole parish, grading from gravel and coarse sand at its base to fine sand, silt and clay at the surface.

• Base and thickness: the base runs from about 125 feet below NGVD 29 in the north to more than 400 feet below it in the south; the aquifer thickens from about 100 feet in the north to almost 400 feet at the southern border.
• Water levels have always moved with the Mississippi's stage. At well PC-49 they fluctuated from 1955 to 2017, at least partly with the river, and fell less than about 10 feet overall.
• Wells (2017): 192 active — 126 irrigation, 43 domestic, 20 industrial, 2 public supply, 1 power generation; 56 to 325 feet deep; yields 8 to 5,400 gallons a minute.
• Use (2014): 28.41 Mgal/d — 15.73 general irrigation, 6.35 aquaculture, 3.29 rice irrigation, 2.94 industry, 0.08 rural domestic, under 0.01 livestock.

The Chicot equivalent system is the "400-foot" and "600-foot" sands of the Baton Rouge area. In the north it merges with the alluvial aquifer, connected to it directly or eroded away and replaced by alluvium. Only one well was registered in it in 2017 — an industrial well in the "600-foot" sand, 360 feet deep, yielding 535 gallons a minute — and in 2014 it supplied 2.25 Mgal/d for industry.

Evangeline equivalent system

The "800-foot", "1,000-foot", "1,200-foot", "1,500-foot" and "1,700-foot" sands of the Baton Rouge area. The system dips south, so its sands progressively meet the alluvial aquifer from below — the deepest, such as the "1,700-foot" sand, reach farthest north. Its base is about 1,600 feet below NGVD 29 in the south of the parish.

• Water levels: in 2001 the "1,200-foot" sand stood from about 20 feet above NGVD 29 in the middle of the parish to about 10 feet below it in the southeast, flowing southeast. At well PC-155 in that sand, levels fell about 20 feet from the 1970s to the 2010s; at PC-39 in the "1,500-foot" sand they varied with the seasons but held steady from the 1950s to the 2010s.
• Wells (2017): 81 active — 36 public supply, 29 domestic, 7 irrigation, 4 industrial, 5 power generation; 290 to 1,482 feet deep; yields 2 to 2,500 gallons a minute.
• Use (2014): about 5.22 Mgal/d — 2.15 general irrigation, 1.86 power generation, 0.86 public supply, 0.17 rice irrigation, 0.12 industry, 0.05 rural domestic, under 0.01 livestock.

Jasper equivalent system

The "2,000-foot", "2,400-foot" and "2,800-foot" sands of the Baton Rouge area.

• The "2,000-foot" sand lies under most of the parish, its base from about 500 feet below NGVD 29 in the north to 1,900 feet below near the southern border, and from under 50 to about 200 feet thick.
• The "2,400-foot" sand also lies under most of the parish, its top from about 500 feet below NGVD 29 in the north to 2,400 feet below in the south; its freshwater part is 0 to 300 feet thick.
• Water levels at PC-66 ("2,000-foot" sand) and PC-70 ("2,400-foot" sand) fell about 80 feet or more from the early 1960s to the 2010s, when they levelled off and began to rise slightly.
• Wells (2017): 105 active — 58 public supply, 39 domestic, 4 industrial, 4 irrigation; 469 to 2,343 feet deep; yields 15 to 1,064 gallons a minute.
• Use (2014): about 4.00 Mgal/d — 2.62 public supply, 1.28 industry, 0.10 rural domestic, under 0.01 livestock.

Water levels in wells PC-49 (Mississippi River alluvial), PC-155 ("1,200-foot" sand), PC-39 ("1,500-foot" sand), PC-66 ("2,000-foot" sand) and PC-70 ("2,400-foot" sand).

Groundwater quality

The state's long-term monitoring sampled 37 Evangeline wells (1940–2006) and 71 Jasper wells (1942–87), compared with the EPA's secondary maximum contaminant levels (SMCLs) — guidelines for taste, colour, staining and similar effects.

Every sample from both systems was within the SMCLs for chloride and sulfate.

Surface water

The parish lies in four subbasins: the Atchafalaya (HUC 08080101) in the west, the Lower Red (HUC 08040301) at the northern tip, the Lower Mississippi–Baton Rouge (HUC 08070100) along the east, and the Lower Grand (HUC 08070300) in the southeast. Because of extensive levees, little water flows in by streams, but the Atchafalaya and Mississippi rivers run along its western and eastern borders. In 2014, 1.59 Mgal/d was taken from smaller waters for aquaculture, with under 0.01 for livestock.

• Atchafalaya and Lower Red. The Atchafalaya River begins near the parish's northern border, where the Red River joins water diverted from the Mississippi. Upstream, the Red River at Alexandria averaged 30,770 ft³/s in 1928–82. The Atchafalaya at Simmesport averaged 218,400 ft³/s in 2009–15, with monthly averages from 354,300 in May to 105,600 in November.
• Lower Mississippi–Baton Rouge. The Mississippi carries drainage from more than 40 percent of the conterminous United States but, behind its levees, drains little of the parish. Near Red River Landing, at the parish's north end, it averaged about 460,000 ft³/s in 1928–76. In 2014, 322.12 Mgal/d was drawn from it for power generation.
• Lower Grand. Bayou Grosse Tete, Bayou Tommy, Cholpe Bayou, Bayou Poydras, Bayou Fordoche, smaller streams and False River, an oxbow lake — a former Mississippi channel cut off around 300 years ago or more. The lake is about 22 miles long and 3,212 acres, averages 21 feet deep with a maximum of 65 feet, and is used for recreation. Little streamflow data exist for these waters.

Surface-water quality

Samples from the Atchafalaya River at Simmesport (1952–77) and the Mississippi River near Saint Francisville (1954–2013) were nearly all below the SMCLs.

pH, calcium, magnesium, sodium and chloride were similar in the two rivers; sulfate, iron and dissolved solids differed more. Five mg/L of dissolved oxygen is generally considered the minimum for a diverse community of freshwater warm-water life, including sport fish.

Sources

• White, V.E., 2020, Water resources of Pointe Coupee Parish, Louisiana: U.S. Geological Survey Fact Sheet 2020–3054, prepared in cooperation with the Louisiana Department of Transportation and Development. https://pubs.usgs.gov/publication/fs20203054
• The tables, figures and captions come from the fact sheet's PDF, which the web version lacks.
• Rewritten in hubnx's own words.