Solute Transport and Filtering Through a Riparian Forest

Abstract

A two and a half year study was conducted to determine the fate of solutes applied to the landscape position just above the upper edge of a forested riparian zone. Nitrate (NO3-N) and bromide (Br) sources were applied to an 18 18 m killed grass sod area at the rates of 188 and 336 kg ha1 N and Br, respectively. Soil samples were collected four times during the study to determine vertical and lateral transport within the treatment area. Shallow groundwater wells were installed at transect positions 13 and 6 m upslope, and 2, 7, 12, 17, and 22 m downslope from the grass sod-riparian forest interface. There were two sets of wells at each transect position in the treatment area, and three sets of wells at each transect position in the forested riparian zone. The well depths for each set were 0.5, 1.0, 1.5, and 2.0 m. Water samples were collected biweekly except for the final six months of the study, when they were collected monthly. Water samples were analyzed for NO3-N and Br content. Mass balance calculations based on soil sample analyses showed that the equivalent of 59 and 78% of the applied NO3-N and Br had left the treatment area after one year. High NO3-N concentrations observed in well waters in the upslope end of the treatment area showed that shallow groundwater entering from upslope was carrying NO3-N in addition to that applied. Comparisons of Br and NO3-N concentrations in the wells and also Br/NO3-N ratios showed that the riparian forest was effectively filtering NO3-N entering from the treatment area. Very little NO3-N was observed in wells 7 and 12 m into the forest. Bromide concentrations observed in the wells were relatively high at 2, 7, and 12 m into the forest. Mean Br/NO3-N ratios showed that some Br had reached as far downslope as 22 m into the riparian forest by the end of the second year of the study. One pulse of Br appeared in the riparian forest 8 to 11 months after application and a second pulse with greater concentrations appeared 16 to 18 months after application. The first pulse is believed to be due to transport via macropore flow, while the second may have been associated with Br initially retained in the soil matrix. Overall the study showed that in areas of the Coastal Plain where shallow lateral water movement is a dominant loss pathway, agrichemicals applied in the spring move downslope during the wet months, and depending on soil type, may take several seasons before completely leaving the upslope area and moving into or through the riparian forest.