Linda F. Benedict, Branch, Bill
The salinity of ground water and surface water used for irrigation will always be a concern for Louisiana farmers. Analyses of long-term data on Red River water quality collected by the U.S. Geological Survey (USGS) and of water samples collected by farmers and county agents and analyzed by the LSU AgCenter indicate the Red River can safely be used as a source of irrigation water. Recent USGS studies of water levels and salinity in the Chicot Aquifer of southwest Louisiana and the Mississippi River Alluvial Aquifer in Franklin Parish indicate little change. The LSU AgCenter will continue to analyze irrigation water samples and strongly encourages growers to use this service. Monitoring of soil salinity will also continue.
Salt is one of Louisiana’s natural resources. We not only mine salt for export and store national strategic oil reserves in salt domes, but we find salt in water pumped from our aquifers. Salt water is present at lower depths in all of Louisiana’s aquifers.
In the mid-1980s, the LSU AgCenter began a water testing service to measure salt levels in irrigation water being used by farmers. County agents and farmers collected and the LSU Agronomy Lab analyzed about 100 samples per year. In the past few years, this number has doubled because of increased awareness and concerns about the salinity of irrigation water. The routine water test package gives the levels of calcium, chloride, iron, mag-nesium, manganese, nitrate, potassium, sodium, sulfur, pH and electrical conductance and classifies the levels from very low to very high.
Irrigation water containing salt can increase soil salinity. Each year, farmers throughout the world lose cropland because of increasing soil salinity. In the United States, as many as 48 million acres of cropland are adversely affected by salt. Farmers in many western irrigation-dependent states routinely apply more irrigation water than the crop needs so that salts in the soil will be leached down below the root zone.
Louisiana crops sensitive to soil salinity include strawberries, black-berries, sweet potatoes, corn, rice, sugarcane, peaches and citrus. Soybean tolerance varies by variety. Ryegrass, wheat, bermudagrass and cotton are highly tolerant of soil salinity.Most irrigation in Louisiana is for rice. Most of the water used for rice irrigation comes from wells drilled in two major aquifers. Many southwest Louisiana rice growers depend on the Chicot Aquifer while those in northeast Louisiana depend on the Mississippi River Alluvial Aquifer. Both aquifers store high-quality water; however, wells near the bottom of the aquifers in some areas can produce high levels of salt.
Because of concerns about irrigation water quality, many farmers have investigated surface water resour-ces, which are rarely affected by salinity and cost less to pump than ground water. The U.S. Geological Survey (USGS) has conducted several studies of ground and surface water in cooperation with the Louisiana Department of Transportation, area farmers, the Rice Research Board and the LSU AgCenter.
Red River salinity
Farmers in the Red River Valley of northwest Louisiana have long been interested in surface water for irrigation. In addition to concerns about ground water quality and the higher cost of pumping, well capacity is frequently inadequate for typical agricultural irrigation applications.
The largest source of surface water in northwest Louisiana is the Red River. Water in the Red River may have come from New Mexico, Texas, Oklahoma or Arkansas before arriving in Louisiana. Efforts to regulate the river’s flow started more than 50 years ago with the construction of numerous water control structures in Texas, Oklahoma and Arkansas. More recently, a series of five locks and dams have been constructed on the Red River in Louisiana. All of these structures help even out the flow. Some were also designed to reduce the amount of salt coming from natural salt deposits, brine seeps and petroleum waste in the upper basin.
LSU AgCenter county agents from Caddo, Red River, Natchitoches and Rapides parishes have submitted numerous samples of Red River water for lab analysis. Samples collected from Red River Parish from 1996 to 1998 indicated total salts ranged from 218 ppm to 937 ppm, with an average value of 451 ppm. Chlorides ranged from 10 ppm to 146 ppm with an average value of 89 ppm. These values are considered low to medium.
In a study that began in 1999, AgCenter personnel sampled a 176-acre cotton field in Red River Parish. The field had never been irrigated. The installed system irrigated 126 acres of the 176-acre field. Soil samples were taken from both irrigated and non-irrigated portions of the field and were analyzed for a wide range of soil chemical parameters, including sodium and total salts.
In 2002, after three years of irrigating cotton, the field was sampled again. The average sodium levels in-creased from 14 ppm to 19 ppm on the non-irrigated area and increased from 20 ppm to 27 ppm on the irrigated area. Average total salts on the non-irrigated area decreased from 173 ppm to 162 ppm and on the irrigated area increased from 178 ppm to 204 ppm. These changes were not considered significant. These sites will be sampled again in 2005.
The U.S. Geological Survey (USGS) reported in 1997 that Red River water quality improves as it flows south and east. Total salts decreased from 367 ppm at Hosston in Caddo Parish to 187 ppm at Simmesport in Avoyelles Parish. Sodium decreased from 66 ppm to 33 ppm and chloride from 91 ppm to 50 ppm at those same sampling sites.
Water from the Red River and from almost any other surface water resource in Louisiana can and should be used for irrigating crops. Water quality for irrigation purposes is usually much better than that of the available ground water in the area. In addition, much less energy is required to lift the surface water, and surface-water pumps are usually much less expensive to install. During a dry growing season, salt and sodium levels in the soil may increase. Winter rains should remove some of the increased salinity.
Chicot Aquifer levels and salinity
Rice has been the main crop grown in southwest Louisiana for generations. Rice requires more water for irrigation than other crops and is more sensitive to salinity than most crops. Much of the water used in southwest Louisiana rice fields comes from the Chicot Aquifer, although some rice is irrigated with surface water.
Because of a drought in 1999 and 2000, the salt content of some of the surface water near the Gulf Coast became too high for irrigating rice. Some growers had to activate old wells, pump more water than usual, limit rice acreage and recycle existing water supplies. This led to concerns over both the levels of water in wells and the salinity of water pumped from the Chicot Aquifer.
In 2000, the LSU AgCenter and the Louisiana Rice Research Board entered into an agreement with the USGS for a three-year study of well water levels and salinity.
The highest water levels in the Chicot Aquifer are usually found in northwest Beauregard Parish wells at about 160 feet above sea level. Lower levels are found in Acadia, Calcasieu and Jefferson Davis parish wells at 50 feet below sea level and in southern Evangeline Parish at 60 feet below sea level.
Ground water generally flows from areas of higher elevation to areas of lower elevation and to areas where water is being heavily pumped. Flow is gener-ally towards rice-growing areas of Acadia, Jefferson Davis, southern Evangeline and eastern Calcasieu parishes. In the northern part of the aquifer, flow is generally towards the south and southeast. In the southern part of the aquifer, flow is to the north, away from coastal areas and toward rice-growing areas. Along the eastern edge of the aquifer, flow is generally to the west.
From 1990 to 2000, the USGS reported water levels in wells dropped from 1 foot per year to 2 feet per year. They found water levels rising slightly between 2000 and 2003. This rise presumably came from higher rainfall in 2002 and 2003, compared with the rainfall in 1998 to 2000, and from reduced pumping because of fewer acres of rice grown in 2002 and 2003.
Seasonal water use for rice irrigation is typically from February to June. The lowest water levels in wells are usually found in June and the highest levels in January. Water levels in the rice-growing areas generally rose less than 15 feet between June 2000 and January 2001. From June 2002 to January 2003, water levels rose as much as 25 feet throughout most of the rice-growing areas.
Water levels were measured hourly at five wells, and the levels in these wells typically declined from 10 to 25 feet. This decline began in February or March and continued through May or June. After June, water levels usually began to rise in recovery. The largest range of water-level fluctuations, about 25 feet, was noted at a well in Jefferson Davis Parish. Water levels fluctuated only about 10 feet annually at wells in Evangeline and Cameron parishes, which are located near the edge of the rice-farming area. From 2001 to 2003, water levels at the five wells often recovered to the same or higher level than in the previous year.
In the Beauregard, Allen and Rapides parish area of the Chicot Aquifer, specific conductance (converted to ppm total salts) is typically below 64 ppm. Values typically increase south and east and generally range from 97 ppm to 320 ppm in rice-growing areas of northern Jefferson Davis, western Evangeline and eastern Lafayette parishes. Total salts generally range from 320 ppm to 640 ppm in most of the rest of the rice-growing areas, except in an area along the border between Calcasieu and Jefferson Davis parishes near Iowa, La., parts of northeastern Cameron Parish, parts of Vermilion Parish, northwestern and central St. Landry Parish and sever-al areas along the eastern boundary of the study area where the Chicot Aquifer system merges with the Atchafalaya River Alluvial Aquifer. In these areas, total salts often exceed 640 ppm and range as high as 7,872 ppm.
More than 500 water samples were analyzed from 164 wells during the 2000 and 2003 seasons in the rice-growing areas of the Chicot Aquifer. Most of the wells were used for irrigation, but some were used for other purposes. Samples were collected by farmers, well owners, county agents and USGS personnel and analyzed at the USGS lab in Baton Rouge. Total salts exceeded 640 ppm in water samples from wells in Cameron, Vermilion, St. Landry, St. Martin and St. Mary parishes and in several wells near Iowa. Total salts exceeded 1,320 ppm in only two wells – an irrigation well about two miles south of Iowa and a USGS observation well used to monitor salt water encroachment in central Vermilion Parish.
One irrigation well in Calcasieu Parish and one in Cameron Parish were checked hourly while pumps were run-ning. Total salts were above 640 ppm in both wells. Salts fluctuated within a range of about 96 ppm at both wells. Levels increased from 32 ppm to 128 ppm during individual pumping events, usually within the first few of hours of pumping, and then usually stabilized with a range of 6 ppm to 13 ppm. No long-term trends in specific conduc-tance, or total salts, were evident in either well over the three-year period of the study.
Groundwater levels and salinity in Franklin Parish
When the LSU AgCenter conducted Futures Forums in all parishes in 1999, residents were asked to identify issues of concern. Franklin Parish residents identified groundwater as one of their primary concerns. USGS data indicated a wedge of salty ground water extending north and south through the Winnsboro area. Water from an irrigation well on the LSU AgCenter’s Scott Research and Extension Center in Winnsboro pro-duces a high salt level. Many farmers in the area have had the same experience.
A water resource committee requested the USGS to study the status of parish ground water levels and salinity. In 2002, the USGS collected and analyzed water samples for salinity from about 25 wells during the peak of the irrigation season. During the off-season, water levels were measured in 75 wells.
Preliminary results indicated little change from previous USGS studies. Specific areas in Franklin Parish have high levels of salinity. Water levels in wells declined slightly during the irrigation season but recovered during the off-season.
(This article appeared in the spring 2004 issue of Louisiana Agriculture.)