Water quality of seasonally flooded agricultural
fields in Mississippi, USA
Jonathan D. Maul
∗, Charles M. Cooper
U.S. Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory, 598 McElroy Drive, Oxford, MS 38655 USA
Received 11 May 1999; received in revised form 17 February 2000; accepted 24 March 2000
Abstract
Planned flooding of agricultural fields is performed to prevent erosion (e.g. sheet, gully, and rill) and provide habitat for waterfowl. As a post-harvest field treatment, flooding is becoming more common in the agriculturally dominated landscape of the Mississippi Alluvial Valley (MAV) in the southeastern United States. Despite this trend, information pertaining to water quality characteristics of water remaining on fields during the winter and subsequent relationships with environmental and biological processes is sparse. Because the water retained on fields is eventually released into adjacent waterways prior to planting, it is critical to monitor water quality parameters of these flooded fields. Water quality parameters of flooded agricultural fields were assessed from January to March and compared to those observed in impounded wetlands. Temporal variation of parameters among sampling dates was also examined. Mean (±S.E.) suspended solids concentration was greater (p<0.05) in flooded agricultural fields (283.3±98.7 mg l−1) than impounded wetlands (79.5±25.3 mg l−1) and an interaction of
habitat and sampling date was detected on dissolved solids concentration (p<0.05). Water temperature, pH, dissolved oxygen,
ammonia, nitrate, total phosphorus, enterococci bacteria, and fecal coliform bacterial concentrations exhibited temporal variation among sampling dates (p<0.05). For both flooded fields and wetlands, fecal coliform and enterococci concentrations
peaked at 2887.5 and 675.0 colony forming units (CFU) 100 ml−1, respectively, during the first sampling date (January) and
declined to 133.2 and 33.3 CFU 100 ml−1, respectively, in March. Results of this study indicated that: (1) flooded agricultural
fields had greater variability of water quality parameters than wetlands; (2) 53% of measured water quality parameters exhibited temporal variation and (3) impounding water may facilitate decreases in bacterial concentrations. Holding water on agricultural fields and knowledge of temporal water quality trends may provide a means to decrease contaminant concentrations, thus improving quality of potential runoff that may enter adjacent bodies of water. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Agricultural fields; Bacteria; Erosion control; Mississippi; Nutrients; Seasonal flooding; Sedimentation; Water quality
∗Corresponding author. Present address: Environmental Sciences
Program, Ecotoxicology Research Facility, Arkansas State Uni-versity, P.O. Box 847, State UniUni-versity, AR 72467, USA. Tel.:
+1-870-972-2570; fax:+1-870-972-2577.
E-mail address:jmaul@navajo.astate.edu (J.D. Maul).
1. Introduction
The Mississippi Alluvial Valley (MAV) is an ex-pansive (10 million ha) intensively farmed area within the southeastern United States characterized by highly erosive alluvial soil. Agricultural production typically includes cotton (Gossypium hirsutumL.), rice (Oryza sativa L.), and soybean (Glycine max L. Merrill). Historically, uncontrolled agricultural runoff
ing storm events transported nutrients and sediments from agricultural fields into receiving streams (Dendy, 1981; McDowell et al., 1981, 1989; Cooper, 1993). Uncontrolled runoff often impacts water quality of receiving systems producing damaging effects on aquatic invertebrate and fish communities (Cooper, 1987, 1988; Cooper and Knight, 1987).
The propensity of retaining winter precipitation on agricultural fields has recently increased in the MAV. Field-flooding offers potential cost savings to land-owners by (1) preventing sheet, rill, and gully erosion; (2) decreasing herbicide application costs and (3) re-taining soil nutrients (McDowell et al., 1989). Further-more, because field-flooding increases availability of high-energy waste grain to wildlife, such actions have been widely used in federal, state, and private wildlife management efforts to sustain non-breeding waterfowl populations (Loesch et al., 1994).
However, little information is available on the qual-ity of water retained on fields. Knowledge of water quality parameters such as nutrients, sediments, and bacterial concentrations of flooded fields is important because the water retained on these fields during the winter is eventually released into receiving streams and rivers prior to planting in early spring. Similar wa-ter retention in small ponds and reservoirs has been shown to maintain water quality downstream by trap-ping sediments (Dendy and Cooper, 1984), transform-ing nutrients (Reddy and Graetz, 1981), and allowtransform-ing for deposition of nutrients via sedimentation (Cooper and Knight, 1990). Knowledge pertaining to dynamics of sediment retention, draw-down, and water quality relationships within flooded agricultural habitats could benefit both landowners and watershed researchers. Thus, the objectives of this study were to (1) com-pare water quality parameters of flooded agricultural fields to those observed within impounded wetlands that contained naturally occurring vegetation and (2) examine temporal variation of water quality parame-ters in flooded fields and compare patterns to those observed within impounded wetlands.
2. Methods
2.1. Study area
The study was conducted in the MAV at five agri-cultural sites and five impounded wetland sites on the
Yazoo National Wildlife Refuge (NWR) and nearby (<5 km) privately owned farmland in Washington Co., MS, USA. Agricultural sites were untilled rice and soybean fields that were flooded by precipitation and ranged in flooded area from 7.8 to 71.1 ha. Slotted-board riser pipes (field-scale water retention/grade control devices) were used to retain water at a depth ranging from 5 to 50 cm with accumulation beginning in late December.
The Yazoo NWR contained a complex of adja-cent impounded wetlands ranging in area from 8.1 to 9.8 ha and water depth from 10 to 75 cm. These wetlands provided a unique comparison to flooded agricultural fields in that they were manipulated hy-drologically and physically in a similar fashion (i.e. drained and tilled on a 1 or 2 year rotation) but native vegetation was permitted to grow for wildlife food. Furthermore, wetlands naturally function to improve water quality (Mitsch and Gosselink, 1993); thus, they are a logical reference or benchmark for flooded agricultural fields. All impounded wetlands were ini-tially flooded by precipitation and pumped well water, and endemic vegetation and algae flourished. Plant species common within impounded wetlands were pondweed (Potamogetonsp.), smartweed (Polygonum sp.), water-milfoil (Myriophyllum sp.), arrowhead (Sagittariasp.), false-loosestrife (Ludwigia sp.), and numerous grasses in drier areas and on levees.
Soils in the project area varied from well-drained medium-textured alluvial deposits to fine textured, poorly drained soils since this region was formed and re-formed by active meanders of the Mississippi River. The flooded fields used in this study were mainly composed of a thermic vertic haplaquepts of poorly drained, fine textured soil named Sharkey-level phase clay.
2.2. Water quality measurements
Water samples were collected from wetlands and flooded agricultural fields approximately every 14 days from 27 January to 10 March 1997 (i.e. the period during which most fields were inundated). Water tem-perature (◦C), pH, dissolved oxygen (mg l−1), conduc-tivity (mS cm−1), and salinity were recorded at each
interface or at a point within the flooded field or wet-land at a water depth of at least 25 cm. Because this was the location where runoff occurs, water quality measurements at this location were probably more in-dicative of what might enter the stream during runoff events than other locations within the flooded area. For samples collected by wading into fields and wetlands, care was taken so as to avoid disturbance of the sample (i.e. minimize sampling bias). All water samples were stored in 1.0 l plastic containers and cooled with ice immediately following collection and during transport to the Water Quality Laboratory at The University of Mississippi. Samples were refrigerated overnight and processed the following day. From water sam-ples, total solids (mg l−1), dissolved solids (mg l−1),
suspended solids (mg l−1), filtered orthophosphate
(mg l−1), total phosphorus (mg l−1), ammonia (NH 3)
(mg l−1), nitrate (NO3) (mg l−1), total chlorophyll
(mg l−1), fecal coliform (colony forming units (CFU) 100 ml−1), and enterococci (CFU 100 ml−1) con-centrations were obtained following APHA (1992) methods. These parameters were chosen because el-evated levels could impact aquatic life. Furthermore, several of these parameters have criterion developed to facilitate assessing impacts to aquatic systems (USEPA, 1986). Coliform and enterococci concentra-tions were investigated because they can indicate the water quality status of aquatic systems.
2.3. Data analysis
Repeated measures analysis of variance (ANOVA) was used to compare water quality parameters be-tween flooded agricultural fields and impounded wetlands (i.e. between habitats) and among sampling dates (PROC GLM, SAS Institute Inc., 1989). Of the original five agricultural fields selected for the study, two were drained midway through the study period for farming preparation. Consequently, these data were excluded from the repeated measures ANOVA because of an incomplete time series. Water qual-ity data were log transformed prior to analyses to satisfy equal variance and normality assumptions of ANOVA (Zar, 1984). A sphericity test indicated that the data had a Type H covariance structure, thus satis-fied the additional assumptions of repeated measures ANOVA. Type III sum of squares and a significance level of 0.05 were used for all ANOVA tests.
3. Results
3.1. General water quality patterns
Dissolved oxygen concentration was lowest during sampling dates associated with high concentrations of suspended solids and often following storm events. Likely, flooded sections of fields received input of decaying plant biomass from their associated agri-cultural watershed, reducing oxygen concentrations. However, dissolved oxygen concentration in either habitat never declined below 4 mg l−1, a lower thresh-old level for supporting aquatic systems (USEPA, 1986). Water temperature and pH were not differ-ent between habitats (p<0.05). Temporal differences were observed, however, with both parameters in-creasing towards the end of the flooded period (early March) (Table 1). Because of the shallow nature of these sites, water temperature was closely correlated to ambient temperatures.
3.2. Effects of habitat on water quality parameters
Suspended and dissolved solids were the only mea-sured water quality parameters that differed between flooded fields and impounded wetlands. Samples from agricultural sites contained greater concentrations of suspended solids (mean±S.E.=283.3±98.7 mg l−1) than impounded wetlands (mean±S.E.=79.5±25.3 mg l−1) (F1,6=5.01, p=0.0067) (Fig. 1) and an
in-teraction effect of habitat and sampling date was detected on dissolved solids (F3,18=4.44,p=0.0168)
(Fig. 2).
3.3. Temporal effects on water quality parameters
Of the fifteen water quality parameters mea-sured, eight exhibited temporal variation over the study period. Water temperature (F3,18=15.37,
p=0.0066), pH (F3,18=7.44, p=0.0019), dissolved
oxygen (F3,18=9.72, p=0.0005), total phosphorus
(F3,18=9.42, p=0.0006), ammonia (F3,18=18.60,
p=0.0001), and nitrate (F3,18=14.41, p=0.0045)
were different among sampling dates (Table 1). In addition, enterococci (F3,18=20.89, p=0.0001) and
coliform concentrations (F3,18=7.25,p=0.0022)
Table 1
Mean (±S.E.) water quality parameters among sampling dates from flooded sites (n=8) in Washington Co., MS, from 27 January to 10 March 1997a
Date pH Water
temp-erature (◦C)
Dissolved oxygen (mg l−1)
NH3 (mg l−1) NO3 (mg l−1) Total phosphorus (mg l−1)
1/27/1997 7.94 a 13.25 a 10.97 a 0.21 a 0.06 bc 0.87 a
(0.11) (1.42) (0.27) (0.12) (0.02) (0.20)
2/10/1997 7.85 a 8.83 a 10.67 a 0.11 a 0.21 a 0.10 c
(0.13) (1.47) (0.19) (0.02) (0.02) (0.02)
2/24/1997 8.22 b 13.80 a 13.93 b 0.07 a 0.05 b 0.31 b
(0.18) (1.27) (0.71) (0.02) (0.01) (0.07)
3/10/1997 8.18 b 19.51 b 9.93 a 0.76 b 0.11 c 0.79 ab
(0.10) (0.52) (1.15) (0.21) (0.02) (0.26)
aParameter means that do not share a common letter among sampling dates (i.e. within columns) are significantly different (p <0.05).
sampling dates (Fig. 3). Suspended and total solids, coliform, and enterococci concentrations were the most variable parameters among agricultural sites.
4. Discussion
4.1. Bacteria
Bacterial concentrations have often been character-ized by extreme variability among and within stream sites, and investigators have had difficulty establishing relationships between such concentrations and other
Fig. 1. Daily precipitation and mean (±S.E.) suspended solids concentration in flooded agricultural fields and impounded wetlands in Washington Co., MS from 1 January to 31 March 1997.
Fig. 2. Mean (±S.E.) dissolved solids concentration among sampling dates in flooded agricultural fields and impounded wetlands in Washington Co., MS from 27 January to 10 March 1997.
In agricultural watersheds, streams are frequently contaminated by bacterial concentrations via agricul-tural runoff (Robbins et al., 1972; Cooper and Mc-Dowell, 1989). Because the flooded sites in this study eventually drain directly into adjacent streams and rivers, they are a potential source of bacterial input into
Fig. 3. Mean (±S.E.) enterococci and fecal coliform colony forming units (CFU) 100 ml−1 in flooded study sites (i.e. wetlands and fields combined) in Washington Co., MS from 27 January to 10 March 1997.
these streams. Thus, the temporal trend observed for fecal coliform and enterococci concentrations (Fig. 3) may provide insight for decisions on timing of draw down.
1979), the temporal patterns observed in this study may be related to waterfowl concentrations in the study area. The greatest waterfowl concentrations at the study sites were observed during January and early February after which concentrations generally decreased (Maul, unpublished data). A dilutional ef-fect from precipitation events may also contribute to decrease bacterial concentrations over time (Cooper and McDowell, 1989). It is encouraging that bacte-rial concentrations in flooded agricultural fields were similar to those observed within impounded wetlands containing endemic vegetation (i.e. lack of a habitat effect). In addition, reduction of bacterial concentra-tions within agricultural fields and wetlands over time validated use of water retention to reduce bacterial contaminants in potential runoff.
4.2. Nutrients
McDowell et al. (1989) found that sediments trans-ported 92.5% of total phosphorus and 81.8% of total nitrogen yields in Mississippi cotton field runoff. In this study, total phosphorus and ammonia concentra-tions were not only different among sampling dates (Table 1), but the greatest concentrations occurred si-multaneous to elevated suspended solid concentrations and rainfall events. This concurs with the expected relationship among these variables (McDowell et al., 1984, 1989). Thus, retaining runoff may potentially sequester a proportion of nutrients on fields via sedi-mentation (Cooper and Knight, 1990). Introduction of lateral runoff from non-flooded portions of sites into flooded portions may have contributed to the temporal patterns observed for nutrient concentrations. Lateral runoff likely transported soluble phosphorus released from post-harvest crop stalks and residue of past fer-tilizer applications. Nitrate concentration was also different among sampling dates; however, it never ex-ceeded the annual mean (±S.E.) of 3.24±0.96 mg l−1
reported by McDowell et al. (1984) for MAV cotton fields. Interestingly, ammonia, nitrate, and phos-phorous concentrations were not different between flooded fields and impounded wetlands.
4.3. Solids
The interaction effect of habitat and sampling date on dissolved solids indicated that concentrations of
dissolved solids varied over time but the pattern was not the same within both habitats (Fig. 2). As might be expected, based on the lack of stabilizing vegetation and field gradation, suspended solid con-centrations were greater in flooded agricultural fields than wetlands (Fig. 1). Within impounded wetlands, high stem density of hydrophytic herbaceous vege-tation may have enhanced sediment deposition (i.e. decreased concentrated flow velocity) (Meyer et al., 1995). Although agricultural fields were partially flooded, much watershed soil remained exposed (up to 30% in some fields) and accumulation of runoff from exposed contiguous areas during storm events likely increased suspended solid concentrations. Peak concentrations of suspended solids were observed in samples collected several days following storm events that produced >6 cm of daily precipitation (Fig. 1) (Climatological station at Belzoni, MS, 47 km east of Yazoo NWR) (NOAA, 1997).
Variability of suspended solids concentration on most sampling dates was greater in agricultural sites than impounded wetlands. Within agricultural sites, variability was greatest when the mean suspended solids concentration was greatest, indicating that some agricultural fields responded differently than others, in terms of suspended solids, and may be related to factors such as adjacent agricultural watershed or per-haps wildlife use. This variability will make it difficult to identify consistent temporal trends for suspended solids in these flooded habitats and may preclude us-ing this technique to improve water quality when not accounting for the effects of a number of additional variables.
5. Conclusions
tem-poral variation. The water retention structures in this study could feasibly function in most deltaic systems. Accumulation of water quality information from sim-ilar studies in other systems is needed to validate the overall usefulness of water retention on agricultural fields as a means to decrease bacterial concentrations and allow for sedimentation. Furthermore, future re-search should examine the relationship between water quality of flooded fields and effects of delayed runoff on animal communities in receiving streams and rivers.
This study suggests that the potential conveyance of low quality runoff and sediment loss from agricultural fields into rivers and streams may be minimized when water is retained on fields and later release is grad-ual over time. Although not tested in this study, major storm events and extensive wildlife use may be impor-tant for predicting patterns of water quality parameters in flooded agricultural fields and should be considered in future studies. Because water quality parameters of flooded agricultural fields were highly variable, it is difficult to generalize a management strategy. How-ever, the observed variability suggests that flooded fields be treated and managed on a site-specific basis whenever feasible.
Acknowledgements
We thank J.L. Farris, M.T. Moore, P.C. Smiley Jr., L. Strong, and S. Testa III for comments on previous drafts of this manuscript; J.A. Kushlan for advisory support; T. Mikuska for field assistance; W.B. Gille-spie Jr. for aid in laboratory analyses; the Durant’s for access to their land; and T. Wilkins and the staff of Yazoo NWR. The U.S. Army Corps of Engineers, Vicksburg District; USDA1-ARS National Sedimen-tation Laboratory, Oxford, MS; and The University of Mississippi provided cooperative funding for the study.
1All programs and services of the U.S. Department of Agricul-ture are offered on a non-discriminatory basis without regard to race, color, national origin, religion, sex, marital status or handi-cap. Names of commercial products are included for the benefit of the reader and do not imply endorsement or preferential treatment by the U.S. Department of Agriculture.
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