How much water Vermilion 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 93.6 million gallons a day (Mgal/d): about 61.9 Mgal/d of surface water and 31.8 Mgal/d of groundwater. Rice irrigation took about 67 percent (62.5 Mgal/d).
| Use (2010, Mgal/d) | Groundwater | Surface water | Total |
|---|---|---|---|
| Public supply | 6.39 | 0.00 | 6.39 |
| Industrial | 1.55 | 0.00 | 1.55 |
| Rural domestic | 2.29 | 0.00 | 2.29 |
| Livestock | 0.07 | 0.27 | 0.33 |
| Rice irrigation | 5.51 | 57.02 | 62.53 |
| General irrigation | 0.11 | 0.44 | 0.55 |
| Aquaculture | 15.83 | 4.14 | 19.97 |
| Total | 31.75 | 61.86 | 93.61 |
By source, all the groundwater came from the Chicot aquifer system; surface water came from Bayou Queue de Tortue (20.18 Mgal/d), the Vermilion River (20.18) and other streams (21.50). Counted every five years since 1960, withdrawals peaked in 1980 at about 548 Mgal/d and have declined steadily since.
Groundwater
The Chicot aquifer system is the main source of fresh groundwater — chloride of 250 milligrams per litre (mg/L) or less; deeper aquifers hold salt water. The base of fresh water lies from less than 300 feet below the National Geodetic Vertical Datum of 1929 (NGVD 29) in the southwest to about 1,000 feet in the northeast. South of White Lake there is no fresh groundwater at all.
Here the Chicot is made of the shallow, upper and lower sands, which dip and thicken to the south. Rain recharges it mainly where it reaches the surface in parts of Allen, Beauregard, Evangeline, Rapides and Vernon Parishes, and a computer model shows more seeping down through the surface clay across southwestern Louisiana, drawn in by heavy irrigation pumping. Wells are its main outflow; every reported groundwater withdrawal in 2010 came from it.
Shallow sand
A clay-and-silt surficial confining layer covers the parish, from less than 50 to more than 400 feet thick. Scattered sand streaks, lenses and layers inside it make up the "shallow sand," mostly less than 100 feet thick. South of Abbeville, near Bancker, it thickens to as much as 250 feet, the clay above thins to less than 20 feet, and the clay below — separating it from the upper sand — is missing or less than 5 feet thick in places. The shallow sand there holds salt water.
In 2010 records listed 2,429 active wells in the shallow sand — 2,132 domestic, 171 irrigation, 37 industrial and 89 public supply — 12 to 350 feet deep (median 100 feet), with yields of 2 to 3,600 gallons per minute.
Upper sand
The upper sand holds fresh water everywhere it occurs except the southwestern corner, where it is only salty; conflicting data suggest a small area near Bancker may lack fresh water too. Only in the north is it fresh through its whole thickness — over most of the parish its fresh water sits on salt water. Its top lies from less than 200 to more than 600 feet below NGVD 29 and its base about 1,200 feet or deeper.
In 2010 records listed 2,701 active wells in the upper sand — 1,992 domestic, 527 irrigation, 114 public supply and 68 industrial — about 60 to 716 feet deep (median 170 feet), with yields of 2 to 5,975 gallons per minute.
Lower sand
A clay from less than 10 to as much as 200 feet thick separates the upper and lower sands. The lower sand is fresh only in the far northeastern corner, along the Lafayette and Acadia Parish lines, where fresh water reaches below 1,000 feet; elsewhere it is salty. No active wells were recorded in it in 2010.
Water levels and salt water
In January 2003 water levels ranged from about NGVD 29 near the coast to about 35 feet below it near the northwestern parish line. Groundwater flowed north in the west of the parish and northwest in the east, drawn by heavy pumping, mostly for rice, here and in parishes to the north and northwest. In the upper sand, levels rise and fall about 1 to 3 feet with the seasons' demand.
Well Ve-637U is screened in the Chicot's upper sand; its land surface is 4.06 feet above NGVD 29 and it is 198 feet deep.
Salt water moving inland is a concern. Before pumping began, groundwater flowed south toward the coast. Pumping in this parish and those to the north has reversed it: groundwater now flows north, giving salt water a gradient to move inland from the coast toward the wells.
Water quality
Samples from 196 wells show very hard water (median hardness 200 mg/L). Iron and manganese generally exceed the EPA's secondary maximum contaminant levels (SMCLs) for drinking water; pH is generally within the 6.5–8.5 range.
| Chicot, 1944–2009 | Median | 10th percentile | 90th percentile | SMCL | Samples within it |
|---|---|---|---|---|---|
| Colour (platinum-cobalt units) | 5 | 0 | 24 | 15 | 82% |
| pH | 7.4 | 7.0 | 7.8 | 6.5–8.5 | 99% |
| Hardness, as calcium carbonate (mg/L) | 200 | 150 | 270 | — | — |
| Chloride (mg/L) | 62 | 18 | 180 | 250 | 100% |
| Iron (µg/L) | 1,200 | 390 | 2,000 | 300 | 8% |
| Manganese (µg/L) | 140 | 64 | 330 | 50 | 8% |
| Dissolved solids (mg/L) | 438 | 280 | 598 | 500 | 71% |
Surface water
About 61.9 Mgal/d of surface water was withdrawn in 2010, about 92 percent of it for rice; the rest went to livestock, general irrigation and aquaculture. The parish's waters include the Gulf of Mexico, the Gulf Intracoastal Waterway, the Mermentau River, Lake Arthur, White Lake and Vermilion Bay. Roughly the western two-thirds drains to the Mermentau basin and the eastern third to the Vermilion. On a coast, which way the water moves decides whether fresh surface water is reliably there: salt water from the Gulf could harm wildlife, freshwater plants and farming, rice and crawfish above all.
Vermilion River
About 20.2 Mgal/d was withdrawn from the river in 2010. At Perry (site 07386980) its average daily flow over 1984–2012 was 1,140 cubic feet per second (737 Mgal/d); it can run upstream during low flow, steady southerly winds or high tides. Samples from 1983–86 a few miles upstream, at State Highway 3073 in Lafayette Parish (site 07386935), show generally soft water within the SMCLs for pH, chloride, sulfate and iron, with a median dissolved oxygen of 5.3 mg/L; 5 mg/L is considered the minimum for a varied community of warm-water life, sport fish included.
Bayou Queue de Tortue
About 20.2 Mgal/d was withdrawn from the bayou in 2010. At Riceville (site 08012300) its average daily flow over 1985–90 was about 364 cubic feet per second, and it too can flow backwards. A few samples from near Rayne, Leleux and Riceville, 1989–2001, show moderately hard to hard water within the SMCLs for pH, chloride, sulfate and iron, but dissolved oxygen generally below 5 mg/L.
| Median (10th–90th percentile) | Vermilion River, 1983–86 | Bayou Queue de Tortue, 1989–2001 |
|---|---|---|
| Dissolved oxygen (mg/L) | 5.3 (3.7–6.9) | 1.8 (0.9–4.9) |
| pH | 7.1 (6.2–7.6) | 7.7 (7.3–8.2) |
| Hardness, as calcium carbonate (mg/L) | 43 (27–62) | 89 (56–140) |
| Chloride (mg/L) | 12 (6–19) | 35 (14–60) |
| Sulfate (mg/L) | 10 (5.2–16) | 4.1 (2.3–8.2) |
| Iron (µg/L) | 190 (140–250) | 20 (9†–80) |
† Estimated: the result was extrapolated outside the calibration curve.
Lakes and the Intracoastal Waterway
Lake Arthur is a shallow widening of the Mermentau River, which feeds and drains it; at its estimated average level of about 4 feet above NGVD 29 it covers about 6 square miles and averages about 7 feet deep. White Lake is natural, 81 square miles and 4.5 feet deep on average, fed mainly by the Old Intracoastal Waterway, which indirectly carries water from the Mermentau River in Cameron Parish. It is now mostly fresh; before control structures were built in the Mermentau Basin during 1950–86, Gulf water made it brackish (1,000 to 10,000 mg/L of dissolved solids).
The Gulf Intracoastal Waterway crosses the middle of the parish from east to west — the largest continuous ship channel on the Louisiana coast. The U.S. Army Corps of Engineers runs the Leland Bowman Lock, the Schooner Bayou Control Structure and the Freshwater Bayou Lock, among others, to steer floodwater, keep fresh water for irrigation and hold back Gulf salt water.
Sources
- Prakken, L.B., and White, V.E., 2014, Water resources of Vermilion Parish, Louisiana: U.S. Geological Survey Fact Sheet 2014–3080, prepared in cooperation with the Louisiana Department of Transportation and Development. https://pubs.usgs.gov/publication/fs20143080
- 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 (public domain) · Adapted from pubs.usgs.gov
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