Effects of Flooding and Drought on Water Quality in Gulf Coastal Plain Streams in Georgia

Since the late 1960s, point-sources of water pollution have been reduced due to their ease of identification and treatment. As water quality problems remain and further point-source measures become less cost-effective, attention is directed toward reducing agricultural nonpoint-sources of P and N. In the past, separate strategies for P and N were developed and implemented at farm or watershed scales. Because of differing biology, chemistry, and flow pathways of P and N in soil, these narrowly targeted strategies may lead to mixed results. In some cases, N management of manures has increased soil P and subsequent P enrichment of surface runoff, while no-till has reduced P losses but increased nitrate leaching. Thus, an integrated approach to nutrient management is needed, with best management practices (BMPs) targeted to critical areas of a watershed that contribute most of the P and N exported. We have developed indices that identify critical sources and transport pathways controlling P and N export. These indices are applied to a mixed land use watershed in Pennsylvania. Areas most vulnerable to P loss are limited to small, well-defined areas of the watershed (<20% of area) near the stream channel. In contrast to P, larger areas contribute to nitrate leaching and generally occur on the upper boundaries of the watershed (60%), where freely draining soils and high manure and fertilizer N applications are made. Thus, differing levels of nutrient management may be appropriate for different areas of a watershed.

Abstract The effect of several cycles of varying length of alternate aerobic and anaerobic conditions on redox potential, organic matter decomposition and loss of added and native nitrogen was investigated under laboratory conditions in flooded soil incubated for 128 days. Redox potential decreased rapidly when air was replaced with argon for the short-time cycles, but decreased more slowly where the aerobic period was long enough to permit build-up of nitrate. The minimum redox potential reached during the anaerobic period was generally lower for the longer cycles, but in all cases was low enough for denitrification to occur. Rate of decomposition of organic matter was faster in the treatments with a greater number of alternate aerobic and anaerobic periods. Total N (native and applied) losses as high as 24.3 per cent occurred in the treatment with the maximum number of cycles and with alternate aerobic and anaerobic periods of 2 and 2 days. Increasing the durations of the aerobic-anaerobic periods decreased the loss of N. A maximum loss of 63.0 per cent of applied 15NH4-N resulted from the shortest (2 and 2 day) aerobic and anaerobic incubation. For soil undergoing frequent changes in aeration status the only labelled N that remained at the end of incubation was found in the organic fraction. Loss of N may have been even greater if labelled inorganic N had not been immobilized by microorganisms decomposing the added rice straw. The greater loss of N resulting from the 2 and 2 day aerobic-anaerobic incubation shows that, in soils where the redox potential falls low enough for denitrification to occur, increasing the frequency of changing from aerobic to anaerobic conditions will increase the loss of N.

Abstract Agroecosystems in the southeastern United States Coastal Plain typically have uplands in agriculture with mixed hardwood forests along the stream channels. This study determined the inputs and outputs of waterborne nutrients for the riparian forest ecosystem of an agricultural watershed. Quantities of phreatic groundwater and precipitation nutrient inputs and phreatic and surface nutrient outputs were determined during 1979. Based on input/output budgets, these streamside forests were shown to be effective in retaining N, P, Ca, and Mg. Partial conversion of the riparian forest to cropland was projected to increase NO3-N and NH4-N loads by up to 800%. Total replacement of riparian forest with crops would increase loads of all nutrients studied except organic N, DMRP, and total P. Land managers can maintain the nutrient filtering capacity of the streamside forest by selective harvesting of hardwoods and by maintaining the present hydrologic regime.

: As part of a basinwide water-quality study, nitrogen and phosphorus data for the Upper Colorado River Basin from the Colorado-Utah State line to the Continental Divide were analyzed for spatial distributions, concentrations associated with various land uses, and temporal trends. Nitrogen and phosphorus concentrations generally increased in a downstream direction. Some nutrient concentrations were elevated at some sites in the upper parts of the basin in areas influenced by increasing urbanization. Sites were grouped according to land use and site type, and median nutrient concentrations were compared among groups. Sites within the agricultural areas of the basin generally had the highest concentrations of nitrogen and phosphorus; concentrations for main-stem, tributary, and urbanization sites were slightly lower than for the agricultural sites. Background sites, or sites with minimal land-use impacts, had very low median nutrient concentrations. Several sites with long-term data were analyzed for temporal trends in concentrations. Several statistically significant downward trends of low and moderate magnitude were observed for nitrogen and phosphorus species. No upward trends were observed in the data at any site.

SUMMARY 1. The Ogeechee River, in south-eastern Georgia, U.S.A, is a blackwater river with an extensive floodplain that is inundated regularly during winter months. Previous studies have shown that low to moderate bacterial production rates cannot support the relatively high suspended bacteria concentrations observed (107−-108 cells ml−1), suggesting an allochthonous source of bacteria. We report the results of a combination of field and flume experiments which demonstrate that river sediments and floodplain soils are significant sources of suspended bacteria during seasonal flooding. Benthic bacteria are also entrained by normal discharges. There are sizeable fluxes of POC and DOC from river sediments and floodplain soils. 2. Bacterial, POC and DOC fluxes (14, 250, and 790 mg Cm−2 h−1, respectively) were substantial when water was percolated upward through floodplain soils. 3. Simulation of overland flow using a flume demonstrated further fluxes of bacteria and POC from floodplain soils (up to 61 and 10700 mg Cm−2h−1, respectively) and river sediments, but did not yield additional DOC from floodplain soils. 4. These laboratory results are supported by experiments in which we measured significant increases in concentrations of bacteria and DOC in a downstream direction in (i) the main river channel during a winter flood in 1986, and (ii) a floodplain slough (channel side-arm) which re-entered the main channel 800m from its initial divergence. Inputs of bacteria and DOC from the surrounding floodplain were estimated to be up to 3500 kg DOC h−1, and 4000 kg bacteria Ch−1 over a 50-km reach. 5. These previously unmeasured fluxes of organic carbon help to explain the high concentrations of suspended bacteria in the Ogeechee River.

The relationship between row crop land use and nitrate N concentrations in surface water was evaluated for 15 Iowa watersheds ranging from 1002 to 2774 km 2 and 10 smaller watersheds ranging from 47 to 775 km 2 for the period 1996 to 1998. The percentage of land in row crop varied from 24 to >87% in the 15 large watersheds, and mean annual NO 3 -N concentrations ranged from 0.5 to 10.8 mg/L. In the small watersheds, row crop percentage varied from 28 to 87% and mean annual NO 3 -N concentrations ranged from 3.0 to 10.5 mg/L. In both cases, nitrate N concentrations were directly related to the percentage of row crop in the watershed (p < 0.0003). Linear regression showed similar slope for both sets of watersheds (0.11) suggesting that average annual surface water nitrate concentrations in Iowa, and possibly similar agricultural areas in the midwestern USA, can be approximated by multiplying a watershed's row crop percentage by 0.1. Comparing the Iowa watershed data with similar data collected at a subwatershed scale in Iowa (0.1 to 8.1 km 2 ) and a larger midcontinent scale (7300 to 237100 km 2 ) suggests that watershed scale affects the relationship of nitrate concentration and land use. The slope of nitrate concentration versus row crop percentage decreases with increasing watershed size.

The eutrophication problem has drawn attention to nutrient leaching from arable land in southern Sweden, and further understanding of spatial and temporal variability is needed in order to develop decision-making tools. Thus, the influence of spatial and temporal variables was analysed statistically using empirical time series of different nutrient species from 35 well-documented catchments (2–35 km 2 ), which have been monitored for an average of 5 years. In the spatial analysis several significant correlations between winter median concentrations and catchment characteristics were found. The strongest correlation was found between inorganic nitrogen and land use, while concentrations of different phosphorus species were highly correlated to soil texture. Multiple linear regression models gave satisfactory results for prediction of median winter concentrations in unmeasured catchments, especially for inorganic nitrogen and phosphate. In the analysis of temporal variability within catchments, internal variables from a dynamic hydrological model (HBV) were linked to concentration fluxes. It was found that phosphorus and inorganic nitrogen concentrations were elevated during flow increase at low-flow conditions, while they were diluted as the wetness in the catchment increased. During unmonitored periods regression models were successful in predicting temporal variability of total phosphorus, phosphate and inorganic nitrogen, while organic nitrogen and particulate phosphorus could not be predicted with this approach. Dividing the data into different flow categories did not improve the prediction of nutrient concentration dynamics. The results and literature review presented, confirm parts of the present HBV-N model approach and will be useful for further development of nutrient routines linked to dynamic hydrological models.

Tables of percentage points for significance tests conrerning the highest or the lowest observation in normal samples, or the two highest or the two lowest observations in normal samples, were recently extended to larger sample sizes than heretofore published and to probability levels of .005 and .001. These tables are being pltblished to satisfy a long existing demand for such extensions.

Principal components analysis (PCA) was performed with water-quality data from studies conducted during 1993 to 1995 to explore potential nitrate-attenuation processes in ground waters of the southeastern USA. Nitrate reduction is an important attenuation process in selected areas of the Southeast. A nitrate-reduction component explains 23% of the total variance in the data and indicates that nitrate and dissolved oxygen (DO) are inversely related to ammonium, iron, manganese, and dissolved organic carbon (DOC). Additional components extracted by PCA include calcite dissolution (18% of variance explained) and phosphate dissolution (9% of variance explained). Reducing conditions in ground waters of the region influence nitrate behavior through bacterially mediated reduction in the presence of organic matter, and by inhibition of nitrate formation in anoxic ground water beneath forested areas. Component scores are consistent with observed water-quality conditions in the region. For example, median nitrate concentration in ground-water samples from the Albemarle-Pamlico Drainage Basin (ALBE) Coastal Plain is {lt}0.05 mg L{sup {minus}1}, median DOC concentration is 4.2 mg L{sup {minus}1}, and median DO concentration is 2.1 mg L{sup {minus}1}, consistent with denitrification. Nitrate reduction does not occur uniformly throughout the Southeast. Median DO concentrations in ground-water samples from the Apalachicola-Chattahoochee-Flint River Basin (ACFB) aremore » 6.2 to 7.1 mg L{sup {minus}1}, and median nitrate concentrations are 0.61 to 2.2 mg L{sup {minus}1}, inconsistent with denitrification. Similarly, median DO concentration in samples from the Georgia-Florida Coastal Plain (GAFL) is 6.0 mg L{sup {minus}1} and median nitrate concentration is 5.8 mg L{sup {minus}1}.« less

A study was conducted to statistically evaluate the effectiveness of riparian buffers for decreasing nitrate concentrations in ground water and for affecting other chemical constituents. Values for pH, specific conductance, alkalinity, dissolved organic carbon (DOC), silica, ammonium, phosphorus, iron, and manganese at 28 sites in the Contentnea Creek Basin were significantly higher (p 20 yr) discharging ground water draining areas with riparian buffers compared with areas without riparian buffers. No differences in chloride, nitrate nitrogen, calcium, sodium, and dissolved oxygen concentrations in old ground water between buffer and nonbuffer areas were detected. Comparison of samples of young (<20 yr) discharging ground water samples from buffer and nonbuffer areas indicated significantly higher specific conductance, calcium, chloride, and nitrate nitrogen in nonbuffer areas. Riparian buffers along streams can affect the composition of the hyporheic zone by providing a source of organic carbon to the streambed, which creates reducing geochemical conditions that consequently can affect the chemical quality of old ground water discharging through it. Buffer zones between agricultural fields and streams facilitate dilution of conservative chemical constituents in young ground water that originate from fertilizer applications and also allow denitrification in ground water by providing an adequate source of organic carbon generated by vegetation in the buffer zone. Based on the median chloride and nitrate values for young ground water in the Contentnea Creek Basin, nitrate was 95% lower in buffer areas compared with nonbuffer areas, with a 30 to 35% reduction estimated to be due to dilution and 65 to 70% due to reduction and/ or denitrification.

To investigate the ability of riparian forest to intercept nutrients leaving adjacent cropland, we examined changes in the chemistry of groundwater flowing from a corn (Zea mays L.) field through a riparian forest. This study provided a comparison to previous studies of a different forest. We sampled groundwater from a transect of wells, and used a Br - tracer to confirm that groundwater moved laterally along the transect through the forest. As groundwater flowed through the forest, NO 3 - concentrations decreased from about 8 mg/L at the edge of the corn held to <0.4 mg/L halfway through the forest. Dissolved organic N and NH 4 + increased by less than 0.1 mg/L, and dissolved organic C did not change with distance [...]

Land use and land cover can have a significant impact on water chemistry, but the spatial scales at which landscape attributes exert a detectable influence on aquatic systems are not well known. This study quantifies the extent of the landscape influence using the proportion of wetlands in the watershed measured at different distances to predict dissolved organic carbon (DOC) concentrations in Wisconsin lakes and rivers, and to determine whether the watershed influence varies with season or hydrologic type of lake. The proportion of wetlands in the total watershed often explained the most variability of DOC in lakes when stepwise regression was used. However, best-model techniques revealed that, for lakes, r2 values often only differed 1–3% between models using the proportion of wetlands in the total watershed and models using only the proportion of wetlands in nearshore riparian areas (25–100 m). In rivers, the proportion of wetlands in the watershed always explained considerably more of the variability in DOC than did the proportion of wetlands in the nearshore riparian zone. The watershed influence also varied seasonally in rivers, as the proportion of the watershed covered by wetlands explained more of the variability in DOC in the fall than in the spring. Overall, the proportion of wetlands in the landscape explained much more of the variability of DOC concentrations in rivers than in lakes.

Riparian (streamside) vegetation may help control transport of sediments and chemicals to stream channels. Studies of a coastal plain agricultural watershed showed that riparian forest ecosystems are excellent nutrient sinks and buffer the nutrient discharge from surrounding agroecosystems. Nutrient uptake and removal by soil and vegetation in the riparian forest ecosystem prevented outputs from agricultural uplands from reaching the stream channel. The riparian ecosystem can apparently serve as both a shortand long-term nutrient filter and sink if trees are harvested periodically to ensure a net uptake of nutrients. (Accepted for publication 28 November 1983)

1. Landscape characteristics of sixty-two subcatchments within the Saginaw Bay Catchment of central Michigan were examined to identify relationships with stream water chemistry. Land use, land cover and elevation were quantified for both entire catchments and the upland–river ecotone (100 m stream buffer strip). Catchment and ecotone data were then empirically compared with stream water chemistry using multivariate and regression analyses. Redundancy analysis was used to partition variance among land use, geology, and the shared influence of land use and geology. 2. Major catchments dominated by rowcrop agriculture had the highest alkalinity, total dissolved solids and nitrate + nitrite concentrations. 3. Strong seasonal differences were observed in total nitrogen and nitrite + nitrate, but not in total phosphorus or suspended solids. Land use and landscape structure factors such as slope and patch density (number of land use patches per km2) accounted for most of the observed variance in summer. 4. In both autumn and summer, landscape factors accounted for much of the observed variation in total dissolved solids and alkalinity. During autumn, geological factors and the shared influence of geology/landscape structure plus land use exerted more influence than did land use alone. 5. Total phosphorus and total suspended solids were much better explained by land use within the stream ecotone in summer than in other seasons. However, total nitrogen, nitrate, orthophosphate and alkalinity were equally well explained by land use within the ecotone and throughout the whole catchment. Only total dissolved solids in summer and ammonium in autumn were explained better by the whole catchment than the ecotone. 6. Our results show that relatively coarse spatial databases can provide useful descriptors of regional water quality.

In low-gradient geologically unconstrained streams, organic particles derived from floodplain soils are an important food source for aquatic life. Since 1993, we have been measuring particulate organic matter (POM) concentration in Ichawaynochaway Creek, a 5 th order blackwater tributary of the lower Flint River, Georgia, USA. Monthly samples have been collected during stable flow periods at 7 stations ranging from near the headwaters downstream to the confluence with the Flint River. Physical measures of channel and floodplain width at each station were used to calculate a floodplain index (floodplain/channel width ratio). POM concentration ranged from 0.46 to 5.65 mg/L and was strongly correlated with floodplain index, being consistently higher in reaches with greater floodplain/channel width ratios. Using streamflow records at one of the sample sites, we found that POM concentration was significantly correlated with daily discharge. This relationship was used to develop a POM rating curve and estimate annual POM transport. During most years, a majority (55-85 %) of annual transport occurred from January through June corresponding with seasonal flood cycles. We used our POM rating curve and the long flow record (continuous since 1943) to investigate several hydrologic scenarios. A hydrologic analysis indicated consistently lower than average spring (March-June) and summer (July-September) daily discharges since the early 1970s. Reduced daily discharges were associated with increasing agricultural water use and a drying trend in regional climate. Our rating curve predicted that POM transport would be substantially reduced in association with declining spring and summer discharges. Reductions in POM availability may cause long-term declines in secondary production, particularly in trophic pathways involving amorphous detritus, non-filtering collectors, and their predators. We also examined how tropical storm frequency might affect POM transport. Years with 2 or more tropical storms had significantly greater summer POM transport than years with 1 or fewer storms. Frequent tropical storms may partially offset regional water use and drying, although it is not clear how climate change will affect storm frequency in the southeast. This study suggests a hierarchy of controls on POM concentration. At the regional scale, hydrology (i.e. magnitude of runoff) is the dominant influence while, at the stream reach scale floodplain geomorphology controls patterns of POM availability. Areas with well-developed floodplains appear to be source areas for organic particles. Management actions or water allocation formulae that systematically reduce the annual period of floodplain inundation may reduce organic particle transport from floodplains. Reductions in organic concentration would mean lower food availability to support aquatic life.