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How much water Jefferson Davis Parish, Louisiana, has, how it is used and how good it is — from a U.S. Geological Survey fact sheet prepared with the Louisiana Department of Transportation and Development, drawing on earlier reports and the USGS National Water Information System.
How the water is used
In 2010 the parish withdrew about 175.06 million gallons a day (Mgal/d): 140.46 Mgal/d of groundwater, all from the Chicot aquifer system, and 34.61 Mgal/d of surface water. Rice irrigation took about 76 percent (133.09 Mgal/d).
| Use (2010, Mgal/d) | Groundwater | Surface water | Total |
|---|---|---|---|
| Public supply | 4.83 | 0.00 | 4.83 |
| Industrial | 0.09 | 0.00 | 0.09 |
| Rural domestic | 0.39 | 0.00 | 0.39 |
| Livestock | 0.16 | 0.00 | 0.16 |
| Rice irrigation | 122.51 | 10.58 | 133.09 |
| General irrigation | 0.69 | 0.46 | 1.15 |
| Aquaculture | 11.78 | 23.57 | 35.35 |
| Total | 140.46 | 34.61 | 175.06 |
Counted every five years from 1960 to 2010, withdrawals peaked in 1980. Rice irrigation alone has ranged from 121.73 Mgal/d in 1960 to 319 Mgal/d in 1980.
Groundwater
The main source of fresh groundwater — chloride of 250 milligrams per litre (mg/L) or less — is the Chicot aquifer system. It holds fresh water down to less than 400 feet below the National Geodetic Vertical Datum of 1929 (NGVD 29) in the parish's northeast corner and to about 900 feet near Jennings. The deeper Evangeline aquifer is fresh to more than 1,500 feet along the northern border and salty everywhere else in the parish; in 2010 state records listed no active wells in it.
How the Chicot moves
In 2003 groundwater in the Chicot flowed generally toward the east-central parish, drawn by heavy pumping, mainly for rice, here and in neighbouring parishes — the system's main outflow. It is recharged by rain soaking in, chiefly north of the parish where it reaches the surface in parts of Allen, Beauregard, Evangeline, Rapides and Vernon Parishes, and by leakage and flow from other aquifers.
In January 2003 water levels ranged from about 25 feet below NGVD 29 in the northwest to about 50 feet below near Jennings. They have generally fallen 50 feet or more since pumping began around 1900, and they swing with the irrigation season — at well JD-9, in the centre of the parish, usually 10 to 20 feet a year. About 96 percent of groundwater withdrawn in 2010 (135.14 Mgal/d) went to agriculture, most of it (122.51 Mgal/d) to rice.
Well JD-9 is screened in the Chicot's upper sand; its land surface is 24.10 feet above NGVD 29 and it is 318 feet deep.
Its layers
The Chicot is a complex stack of unconsolidated gravel, sand, silt and clay that thickens and dips south at about 30 feet per mile. Here it is mostly massive beds of coarse sand and gravel separated by fairly thin clay. A clay surficial confining layer covers the whole parish, from 40 feet thick in the north to as much as 240 feet in the southwest corner.
- Shallow sand — scattered sand streaks and lenses inside the surficial clay. In 2010 records listed 20 active wells, all domestic, 13 to 105 feet deep (median 22 feet); no withdrawals were reported.
- Undifferentiated, upper and lower sands — below the clay. The undifferentiated sand is one massive sand in the northern third of the parish; in the southern two-thirds a clay layer generally splits it into upper and lower sands. The base of the system lies from about 400 feet below NGVD 29 in the northeast corner to more than 1,000 feet at the southern boundary.
Salt water intrudes in two places. The lower sand passes from fresh to salt water at about 600 to 800 feet below NGVD 29 in the centre of the parish. Salt water also sits at the base of the upper sand, about 170 to 360 feet down, in the west of the parish and in eastern Calcasieu Parish near Iowa, where well water has held 100 to 550 mg/L of chloride.
In 2010 records listed 1,761 active wells in these sands — 1,052 domestic, 608 irrigation, 69 public supply and 32 industrial. Eighty-two percent were screened in the upper sand. They were 60 to 760 feet deep (median 185 feet) and reported yields of 4 to 6,000 gallons per minute.
Water quality
The three sands hold water of generally similar quality. Samples from 223 wells show it is generally moderately hard and within the EPA's secondary maximum contaminant levels (SMCLs) for chloride, colour, pH and dissolved solids — but generally above them for iron and manganese.
| Chicot, 1939–2009 | Median | 10th percentile | 90th percentile | SMCL | Samples within it |
|---|---|---|---|---|---|
| Colour (platinum-cobalt units) | 10 | 3 | 55 | 15 | 75% |
| pH | 7.2 | 6.5 | 8.0 | 6.5–8.5 | 88% |
| Hardness, as calcium carbonate (mg/L) | 110 | 62 | 168 | — | — |
| Chloride (mg/L) | 49 | 30 | 115 | 250 | 100% |
| Iron (µg/L) | 1,700 | 410 | 5,000 | 300 | 9% |
| Manganese (µg/L) | 280 | 130 | 400 | 50 | 0% |
| Dissolved solids (mg/L) | 285 | 213 | 432 | 500 | 89% |
Surface water
In 2010 about 34.61 Mgal/d of surface water was withdrawn, for rice, general irrigation and aquaculture. The parish lies in the Calcasieu-Mermentau basin, split here into three subbasins:
- Mermentau Headwaters (northeast), which drains into the Mermentau subbasin. Its Bayou Nezpique joins Bayou Plaquemine and Bayou des Cannes from Acadia Parish to form the Mermentau River near the town of Mermentau. Near Basile (site 08012000) its mean daily flow was 797 cubic feet per second over 1938–2013.
- Mermentau (centre and south), the source of every withdrawal from a named water body — 19.18 Mgal/d: Bayou Chene 9.60, Bayou Lacassine 6.22, the Mermentau River 3.34 and West Bayou Grand Marais 0.02. Another 15.42 Mgal/d came from unnamed streams.
- Upper Calcasieu (northwest), draining generally southwest, with Bayou Serpent and the Calcasieu River. Near Kinder (site 08015500) the Calcasieu's mean daily flow was 2,540 cubic feet per second over 1922–2013.
Fresh water is a concern in the Mermentau subbasin, which drains south through Calcasieu, Cameron and Vermilion Parishes to the Gulf of Mexico. Wind and tide can reverse its flow in the south of the parish, and control structures in those three parishes keep Gulf salt water out. Bayou Lacassine near Lake Arthur (site 08012470) averaged about 545 cubic feet per second over 1987–2005, and the Mermentau River at Mermentau (site 08012150) about 2,243 over 1989–2003; both sometimes flow backwards.
Lake Arthur is a shallow widening of the Mermentau River, which both feeds and drains it. Its level varies several feet a year; at its estimated average of about 4 feet above NGVD 29 it covers about 6 square miles and averages about 7 feet deep.
Water quality
Both streams generally carry soft water within the SMCLs for pH, chloride, sulfate and iron, but dissolved oxygen is generally below 5 mg/L — the minimum considered necessary for a varied community of warm-water life, sport fish included. Lake Arthur's water is generally like the Mermentau River's.
| Median (10th–90th percentile) | Bayou Lacassine near Lake Arthur, 1989–2004 | Mermentau River at Mermentau, 1979–2011 | SMCL |
|---|---|---|---|
| Dissolved oxygen (mg/L) | 4.0 (0.8–7.0) | 4.0 (1.1–7.0) | — |
| pH | 7.2 (6.7–7.8) | 7.1 (6.2–7.6) | 6.5–8.5 |
| Hardness, as calcium carbonate (mg/L) | 43 (28–66) | 40 (17–77) | — |
| Chloride (mg/L) | 22 (9.6–76) | 17 (6.7–37) | 250 |
| Sulfate (mg/L) | 4.6 (2.3–11) | 4.2 (2.3–8.4) | 250 |
| Iron (µg/L) | 110 (less than 30–290) | 170 (20–340) | 300 |
Sources
- White, V.E., and Prakken, L.B., 2014, Water resources of Jefferson Davis Parish, Louisiana: U.S. Geological Survey Fact Sheet 2014–3074, prepared in cooperation with the Louisiana Department of Transportation and Development. https://pubs.usgs.gov/publication/fs20143074
- The tables and figures come from the fact sheet's PDF, which the web version lacks.
- Rewritten in hubnx's own words.
Licence : CC0 1.0 (domaine public) · Adapté de pubs.usgs.gov
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