Collection
of Surface Water Quality Data
Each site has four field ditches which drain to a main channel. Flashboard
riser type water control structures were installed in the field ditches
to restore wetland hydrology and provide a convenient method for measuring
outflow. Surface water samples were collected from the four field ditch
outlets and several locations along the main channel beginning at the onset
of the study and provided a means of monitoring surface water quality at
the sites.
Provided surface flow was occurring, surface water grab samples were collected
on a weekly basis. Duplicate samples were taken from each location; one
sample being acidified for nutrient analysis, the other non-acidified for
determining sediment content. Replicate samples were also taken periodically
to insure quality control. Samples were brought back from the field sites
in iced coolers and refrigerated until analyzed. In the laboratory, samples
were analyzed for nitrate, ammonium, total kjeldahl nitrogen, total phosphorous,
orthophosphorous, and sediment.
Automated samplers were installed on the field ditches for more frequent
monitoring of drainage from the restored wetland plots. The automated samplers
collected samples every 24 hours provided there was flow occurring over
the outlet weirs. Bottles were pre-acidified and the samples were collected
weekly for laboratory analysis. Samplers were not installed at the Craven
county site due to the lack of surface runoff from the site.
Collection of Groundwater Quality Data
Groundwater samples were also collected at the sites. Sampling wells were
installed at depths of 0-30, 30-60, and 90-120 cm along the instrumentation
transect nearest to the water control structures. On the "smooth" microtopography
treatments, two clusters of wells, each consisting of three wells installed
at the three given depths, were installed on each experimental plot. On
the "rough" microtopography treatments, four clusters of wells were installed
on each experimental treatment plot, two clusters being placed on high
ridged areas, the other two being placed in furrowed depressional areas.
Sampling was conducted on a regular basis, provided that the water table
was sufficiently high enough to collect a majority of samples.
Groundwater samples were collected bi-monthly throughout the study period, provided the water table was sufficiently high to allow collection of a majority of samples. The sampling wells were fully purged approximately 24 hours prior to sampling. The sampling apparatus was flushed with deionized water before collection of each sample. Replicate samples and sample blanks, containing deionized water pumped through the sampling apparatus, were taken to insure quality control. Groundwater samples were analyzed for nitrate, ammonium, total kjeldahl nitrogen, total phosphorous, and orthophosphorous.
The two field sites have responded differently to restoration treatments. The hydrologic studies conducted at the sites indicated that wetland hydrology has been restored to the Beaufort county site, but not to the Craven county site. Field experiments conducted at the Craven site during the spring of 1997 revealed the presence of many large preferential flow channels, mostly due to rotted tree roots. The channels act as a drainage network allowing water to move quickly through the soil profile. During the 1996 growing season (March 30 to November 11), the water table at the Beaufort and Craven county sites remained within 30 cm of the soil surface for approximately 185 and 35 days, respectively. The dryer conditions of the Craven county site have resulted in fewer water quality samples being collected as compared to the Beaufort county site.
Surface Water Sampling: The Beaufort County Site
Approximately 350 surface water grab samples were collected from both sites during the period from December 1995 through July 1997. Total nitrogen concentrations in surface water samples collected from the Beaufort county site typically ranged from 0.75 to 1.5 mg/L. Total nitrogen concentrations in runoff are generally higher from the wetland field plots than from the agricultural fields near the site, possibly due to the higher levels of organic nitrogen from the wetland plots. Plant residues are not removed from the wetland plots as they are on plots in agricultural production, which may explain the higher organic nitrogen concentrations observed from the wetland plots. Nitrate concentrations in drainage from wetland plots were typically below 0.1 mg/L. However, higher concentrations up to 1 mg/L were measured following heavy rainfall events during dry periods. Runoff from the agricultural fields contained slightly higher concentrations of nitrate than the wetland plots, most likely due to fertilizer applications and the lower water table conditions imposed by agricultural drainage. Ammonium concentrations typically ranged from 0.1 to 0.3 mg/L at the Beaufort county site. Following heavy rain events, the wetland plots yielded ammonium concentrations as high as 2.0 mg/L.
Total phosphorous concentrations in surface waters typically ranged from 0.1 to 1.0 mg/L at the Beaufort county site, much of which was in the soluble form. Total phosphorous concentrations were generally higher in the drainage from the wetland field plots than the agricultural plots. Insoluble phosphorous concentrations were similar between the wetland and agricultural plots, with 0.1 to 0.2 mg/L being a representative range.
Concentrations of sediment in surface water samples collected from the Beaufort county site typically ranged from 0 to 20 mg/L. Sediment concentrations were highest from sampling locations near agricultural plots and lowest from the restored wetland field plots. Sediment concentrations decrease with distance of flow down the main channel, due to settling with decreased water velocity.
Surface Water Sampling: The Craven County Site
Total nitrogen concentrations from surface water samples collected from the Craven county site commonly ranged between 0.5 to 1.8 mg/L. Total nitrogen was highest in samples collected from the main channel at the site, due to greater amounts of nitrate. Nitrate concentrations in the main channel varied between 0.5 and 1.0 mg/L on average, with an apparent reduction in nitrate with distance traveled down stream. Nitrate concentrations in drainage from the restored wetland site and from the forested ditches were much lower, generally less than 0.1 mg/L. Ammonium concentrations were similar to those measured at the Beaufort county site, with occasional concentrations as high as 2 mg/L during heavy rainfall events.
Total phosphorous levels at the Craven county site were consistently low, generally less than 0.05 mg/L, except for the six samples collected from a field ditch located next to an unpaved road. This field ditch only experienced outlet flow during periods of heavy rainfall. The mean total phosphorous concentration from this ditch was 0.25 mg/L, approximately 60% of which was in the soluble form. On average, orthophosphorous accounted for approximately 30 to 40% of the total phosphorous amounts measured from the main channel and forested ditches.
In contrast to the Beaufort county site, sediment concentrations in surface water samples collected from the Craven county site tended to increase slightly in the down stream direction. This is believed to be due to stream bank erosion, since stream velocity was observed to increase from upstream to downstream. Sediment concentrations of 0 to 20 mg/L were typical for samples collected from the main channel.
Groundwater Sampling: The Beaufort County Site
Approximately 630 groundwater samples were collected from the Beaufort county site for the period from December 1995 through July 1997. Nitrate concentrations in groundwater were found to be as high as 14.2 mg/L (May 6, 1997) from wells located within 90 cm of the soil surface. However, concentrations this high were uncommon, with values between 0.0 and 1.0 mg/L being more representative over the entire study period. Ammonium concentrations were consistently low, generally 0.5 mg/L or less. Total kjeldahl nitrogen values were commonly less than 2 mg/L, except for samples taken from the high ridged areas within the "rough" microtopographical areas, where values ranging from 2 to 5 mg/L were common. Total phosphorous values were generally less than 0.5 mg/L, except on February 8, 1996 when the average total phosphorous concentration in groundwater was 1.1 mg/L with 6 concentrations higher than 3.0 mg/L. Most phosphorous was found to be in the soluble form. Groundwater pH values were rather consistent throughout the study period, commonly ranging between 7 and 8, with values as high as 8.5 and low as 5.0.
Groundwater Sampling: The Craven County Site
Approximately 440 groundwater samples were collected from the Craven county site for the period from December 1995 through July 1997 . Though the same number of sampling wells were installed at each site, fewer samples were collected from the Craven county site due to the lower water table conditions. Samples collected from the site showed nutrient concentrations similar to the groundwater samples collected at the Beaufort county site. Nitrate concentrations from the shallower wells were found to be slightly higher, generally ranging between 0 and 2 mg/L. Nitrate concentrations in groundwater greater than 10 mg/L were also recorded at the Craven site on December 21, 1995. Ammonium concentrations ranged between 0.0 and 0.5 mg/L, while total kjeldahl nitrogen was generally less than 2 mg/L. Phosphorous concentrations were higher at the Craven site than the Beaufort site, generally 1.5 mg/L or less, with most being in the soluble form. The pH of groundwater at the Craven site was found to be more acidic and more variable than the Beaufort site, with values as high as 8.1 and low as 3.4, and generally decreasing with depth of sample.
The Effects of Microtopography on Groundwater Quality
Total nitrogen values were generally highest on the ridged areas within the "rough" microtopography plots. Nitrate, ammonium, and organic nitrogen concentrations were all higher in these areas, relative to other areas within the restored wetland plots. This was due to soil disturbance which occurred during the "roughing" process. Tillage operations, used to create some depressional areas and some ridged areas, removed some or all of the nutrient rich topsoil from the furrowed areas and redeposited it in the ridged areas. Therefore, groundwater nutrient concentrations were highest in the ridged areas due mainly to the presence of greater amounts of nutrients in the soil deposited in these areas. These effects from soil disturbance, as well as those due to variable soil moisture conditions, have created greater variation in the nutrient concentrations in the "rough" microtopography treatments.