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Environmental Setting, Water Quality, and Ecological Indicators of Surface-Water Quality in the Mermentau River Basin, Southwestern Louisiana, 1998-2001 By Stanley C. Skrobialowski, Scott V. Mize, and Dennis K. Demcheck ABSTRACT The U.S. Geological Survey collected data from 29 wells and 24 surface-water sites in the Mermentau River Basin, 1998-2001, to better understand ground-water and surface-water quality; aquatic invertebrate communities; and habitat conditions, in relation to land use. This study was a part of the National Water-Quality Assessment Program, which was designed to assess water quality as it relates to various land uses. Water-quality data were evaluated with criteria established for the protection of drinking water and aquatic life, and bed-sediment data were compared to aquatic-life criteria. Water-quality and ecological data were analyzed statistically in relation to drainage area and agricultural land-use intensity. Concentrations of nutrients and major inorganic ions in ground water and surface water generally were highest in the southeastern part of the study area where soils contain thick loess deposits. Peak concentrations of nutrients in surface water occurred March-May at two sites with high agricultural intensity; the lowest concentrations occurred August-January. The greatest potential for eutrophic conditions in surface water, based on nutrient concentrations, existed March-May, at about the same time or shortly after ricefields were drained. Secondary Maximum Contaminant Levels established by the U.S. Environmental Protection Agency (USEPA) were exceeded for sulfate, chloride, iron, or manganese in samples from 20 wells, and for iron or manganese in samples from all surface-water sites. Fewer pesticides were detected in ground water than in surface water. In 11 of the 29 wells sampled, at least one pesticide or pesticide degradation product was detected. The most frequently detected pesticides or pesticide degradation products in ground water were the herbicides bentazon and atrazine. Concentrations of 47 pesticides and degradation products were detected in sur- 1 face water. At least 3 pesticides were detected in all surface-water samples. In 72 percent of the samples at least 5 hydrophylic pesticides were detected, and in more than 70 percent of the samples at least 3 hydrophobic pesticides were detected. Although atrazine concentrations in three samples collected in the spring exceeded 3 µg/L (micrograms per liter), the USEPA Maximum Contaminant Level of 3 µg/L was not exceeded because it is based on an annual average of quarterly samples. Concentrations larger than 3.0 µg/L were not detected in samples collected during other times of the year. Tebuthiuron was detected at all surface-water sites; the largest concentration (6.33 µg/L) was detected at a site on Bayou des Cannes, and was the only detection that exceeded the criterion (1.6 µg/L) for the protection of aquatic life. Malathion was detected at 16 surface-water sites; the largest concentration (0.113 µg/L) was detected at a site on Bayou Lacassine, and was the only detection that exceeded the criterion (0.1 µg/L) for the protection of aquatic life. Concentrations of fipronil exceeded numeric targets for acute total maximum daily loads (2.3 µg/L) at 3 sites and chronic total maximum daily loads (4.6 µg/L) at 14 sites. Maximum pesticide concentrations in surface water usually occurred in the spring at about the same time or shortly after ricefields were drained. Concentrations of DDE in bed sediment at two sites exceeded interim freshwater sediment quality guidelines for the protection of aquatic life. Fipronil sulfide and desulfinylfipronil were detected at all 17 sites from which bed-sediment samples were collected, but there are no current (2002) guidelines with which to evaluate the environmental effects of fipronil and degradation products. Two methods were used to group the ecological data-collection sites: (1) Sites were grouped before data collection (according to the study design) using drainage
Page 1 and 2: National Water-Quality Assessment P
Page 3 and 4: Environmental Setting, Water Qualit
Page 5: FOREWORD The U.S. Geological Survey
Page 8 and 9: FIGURES 1-4. Maps showing: 1. Locat
Page 12 and 13: area and agricultural land-use inte
Page 14 and 15: 4 30° 30´ 30° 00´ 93° 00´ VER
Page 16 and 17: Table 1. Surface-water and ecologic
Page 18 and 19: 8 30° 00´ 30 ° 30´ 93° 00´ VE
Page 20 and 21: QUANTITY, IN MILLION GALLONS PER DA
Page 22 and 23: aquatic life is exposed to both gro
Page 24 and 25: were sampled monthly November 1998-
Page 26 and 27: Table 3. Drinking-water regulations
Page 28 and 29: and manganese were exceeded in surf
Page 30 and 31: the median concentration of total n
Page 32 and 33: Photograph by Dennis K. Demcheck, U
Page 34 and 35: Major Inorganic Ions and Trace Elem
Page 36 and 37: criteria for the protection of drin
Page 38 and 39: 28 CONCENTRATION, IN MICROGRAMS PER
Page 40 and 41: ank stability at Tortue and the red
Page 42 and 43: Table 9. Median aquatic invertebrat
Page 44 and 45: habitat conditions associated with
Page 47 and 48: invertebrate species in relation to
Page 49 and 50: Table 13. Median aquatic invertebra
Page 51 and 52: Median abundance and richness as me
Page 53 and 54: Water-quality data were compared to
Page 55 and 56: Demas, C.R., Demcheck, D.K., Anders
Page 57 and 58: Sandstrom, M.W., 1995, Filtration o
Page 59 and 60: APPENDIXES 1. Complete names for su
Page 61 and 62:
Appendix Appendix 2. 2. Pesticides
Page 63 and 64:
Appendix 2. Pesticides and degradat
Page 65 and 66:
Appendix 3. Concentrations of fipro
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
61 Appendix 5. Mean quantitative ha
Page 73 and 74:
63 Appendix 6. Classification ranki
Page 75 and 76:
Appendix 8. Taxa and densities of r
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
Page 83:
73 Appendix 9. Loadings of environm
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