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From a U.S. Geological Survey fact sheet of 2021, prepared for water managers and residents.
In 2014, St. Martin Parish, Louisiana, withdrew about 46.99 million gallons of water a day (Mgal/d), including about 35.91 Mgal/d of groundwater.
How the water is used
| Use (2014) | Mgal/d | Share of total |
|---|---|---|
| Aquaculture | 32.28 | agriculture, together about 88% (41.16 Mgal/d) |
| Rice irrigation | 6.44 | |
| General irrigation | 2.38 | |
| Livestock | 0.06 | |
| Public supply | 4.83 | about 10% |
| Rural domestic | 0.81 | about 2% |
| Industry | 0.18 | less than 1% |
Water-use data gathered every 5 years from 1960 to 2010, and in 2014, show withdrawals peaked in 1985 at more than 68 Mgal/d.
Water withdrawals in St. Martin Parish, 1960–2014. Credit: U.S. Geological Survey.
The parish and its aquifers
St. Martin Parish has two separate sections, Upper and Lower, divided by Iberia Parish.
The study area, St. Martin Parish, Louisiana. Base modified from U.S. Geological Survey digital data. Credit: U.S. Geological Survey.
- The Atchafalaya aquifer is the main aquifer. It overlies, or cuts into, the Chicot aquifer system; the two are directly connected and usually treated as one unit. Above them, clay, silt and sand form a surficial confining unit that limits water seeping down from the surface.
- The Evangeline aquifer lies beneath but holds little fresh groundwater here.
- Depth of fresh water. In Upper St. Martin Parish the base of fresh groundwater is generally 200 to 900 feet below the NGVD 29 datum, deepening from east to west, and reaches about 2,000 feet in a small area near Pointe Coupee and St. Landry Parishes, though intermediate sands there hold salt water. In Lower St. Martin Parish fresh groundwater reaches 200 to 300 feet down, and is absent in the southwest.
Idealised west-to-east hydrogeologic section through St. Martin Parish. Credit: U.S. Geological Survey.
The Atchafalaya aquifer
Made of gravelly deposits that filled scours in the Mississippi River floodplain, it is hard to tell apart from the Chicot below. Its confining layer is 40 to 160 feet thick in Upper St. Martin Parish, thickening from northwest to southeast; its approximate western limit is near the western edge of Upper St. Martin Parish.
- Water levels follow the stage of the Atchafalaya River and may be lowered locally by pumping for crawfish farming, whose ponds are flooded each year and kept full from September to May. Measurements at 27 wells in 1990–92 ranged from 15 feet above to 24 feet below NGVD 29, and well SMn-103 rose and fell about 5 feet with the seasons.
- Wells. In 2017 there were 1,272 active wells: 968 domestic, 206 irrigation, 68 public-supply and 30 industrial, 10 to 280 feet deep, yielding 3 to 5,000 gallons a minute.
- Use. About 31.70 Mgal/d in 2014, mostly for aquaculture (29.05 Mgal/d).
The Chicot aquifer system
Thick sand and gravel, thickening towards the Gulf, beneath most of the parish. Its base lies from about 300 feet below NGVD 29 in the northeast of Upper St. Martin Parish to about 1,130 feet at its south-central border. Where clay divides it, it has upper and lower sand units.
- Water levels. Water moves from the Atchafalaya Basin towards pumping centres in St. Landry, Evangeline and Acadia Parishes, where levels had fallen to 70 feet below NGVD 29 by June 2000. Well SMn-134B, in the lower sand, shows seasonal swings and a long-term decline of about 0.2 foot a year from 1975 to 2015. There is no fresh groundwater in the Chicot in Lower St. Martin Parish.
- Wells. 359 active wells in 2017 — 270 domestic, 57 public-supply, 24 irrigation and 8 industrial — 65 to 912 feet deep, yielding 5 to 4,050 gallons a minute; 348 draw from the upper sand.
- Use. About 4.22 Mgal/d in 2014, about 93 percent (3.91 Mgal/d) for public supply.
Water levels in wells SMn-103 (Atchafalaya aquifer) and SMn-134B (Chicot aquifer system, lower sand). Credit: U.S. Geological Survey.
Groundwater quality
Samples from 33 wells in the Chicot's lower sand (1963–2010) and 29 in the Atchafalaya aquifer (1941–83):
| Chicot (lower sand) | Atchafalaya | |
|---|---|---|
| Median hardness | 36.5 mg/L — soft | 240 mg/L — very hard |
| Median chloride | 185 mg/L | 16 mg/L |
| Iron above its secondary standard (300 µg/L) | about half of samples | all samples |
| Manganese above its secondary standard (50 µg/L) | about half of samples | 83% of samples |
| Dissolved solids above 500 mg/L | 69% | 23% |
Median pH and sulfate in both were within or below the EPA's secondary standards.
Surface water
Three main subbasins drain the parish southward — the Atchafalaya, Bayou Teche and Vermilion — with a small area of Lower St. Martin Parish in the West Central Louisiana Coastal subbasin. Surface-water withdrawals in 2014 were about 5.93 Mgal/d for rice irrigation, 3.23 for aquaculture, 1.90 for general irrigation and 0.01 for livestock.
- Atchafalaya subbasin — more than a third of Upper St. Martin Parish (the east) and most of Lower. The Atchafalaya River and its swamps support fishing, hunting and other activities, and its floodplain carries Mississippi River floodwater through locks, pumping stations and drainage structures; an average of 25 percent of the Mississippi's flow (1984–2007) is diverted into the basin through the Old River Control Structure. Spring floods sustain the wetlands' plants and wildlife. At Simmesport, the river's daily average flow for 1963–2016 was 225,520 cubic feet per second, with the highest monthly average in May (342,173) and the lowest in September (108,526).
- Bayou Teche subbasin — about the central third of Upper St. Martin Parish. At Keystone Lock and Dam near St. Martinville, Bayou Teche averaged 478 cubic feet per second in 1960–2014 (highest in January, 697; lowest in August, 336); 2.29 Mgal/d were withdrawn from it in 2014 (1.48 for rice, 0.81 for aquaculture).
- Vermilion subbasin — along the western edge of Upper St. Martin Parish.
Surface-water quality (Bayou Teche 1965–98; Atchafalaya River at Krotz Springs 1952–72): median pH, chloride, sulfate and dissolved solids were within or below secondary standards. Bayou Teche was soft and the Atchafalaya hard. Bayou Teche's median dissolved oxygen was 6.0 mg/L — above the 5 mg/L considered the minimum for diverse warm-water life including sport fish. The colour standard (15 platinum-cobalt units) was exceeded in 92 percent of Bayou Teche samples and 39 percent of Atchafalaya samples, and manganese in Bayou Teche was within its standard in 58 percent of samples (median 47 µg/L).
Sources
- Lindaman, M.A., and White, V.E., 2021, Water resources of St. Martin Parish, Louisiana: U.S. Geological Survey Fact Sheet 2021–3007. https://doi.org/10.3133/fs20213007
- Water-use figures are from Collier (2018) and Sargent (2011); well records from the Louisiana Department of Natural Resources (2017).
- The source gives 2014 surface-water withdrawals as 11.08 Mgal/d in its table and 11.07 in its text, so no total is given here.
- The map, figures and table values are taken from the fact sheet's PDF.
- Rewritten in hubnx's own words.
Licenza: CC0 1.0 (pubblico dominio) · Tratto da pubs.usgs.gov
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