Assessing Nutrient Transport Following Dredging of Agricultural Drainage Ditches

Authors: Smith, Douglas; Huang, Chi Hua

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2010
Publication Date: 4/26/2010
Citation: Smith, D.R., Huang, C. 2010. Assessing Nutrient Transport Following Dredging of Agricultural Drainage Ditches. Transactions of the American Society of Agricultural and Biological Engineers. 53:429-436.

Interpretive Summary: In many areas of the country, drainage ditches are necessary for agriculture to exist, otherwise, the fields would be too wet to support the crops. In areas where the soils or ditch banks are erodible, sediment buildup requires that dredging occur every 5 to 10 years. Our previous research has shown that immediately after dredging, nutrient transport is much greater in the ditches than prior to dredging. In this study, we examined six years (2003 – 2008) of intensive water quality samples on two agricultural drainage ditches. During this period, one of the ditches had two dredging operations. Using this dataset, we observed removal of nutrients from the water during the 12 months following the dredging (207 lb of NH4-N removed; 14.5 lb of soluble phosphorus removed; and 12 lb of total phosphorus removed). The removal of nutrients from the water resulted in better water quality conveyed to downstream water bodies when considering either the entire year after dredging, or the individual months – up to 12 months after the dredging activity. We attribute this improvement in water quality to: 1) an improvement in the physicochemical status of the sediments after being exposed, 2) “fresh” sediments that were deposited in the ditch bed after dredging, 3) regrowth and recolonization by the algae and plants, and 4) formation of microbial biofilms on sediments and rocks in the ditch bed. To avoid the detrimental impacts on water quality immediately after dredging, and to maximize the benefit of ditch recovery, we propose that ditch managers work with agricultural producers to delay nutrient applications to adjacent fields for at least one month after dredging activities. This should allow the ditches sufficient time to recover their ecological function following dredging.

Technical Abstract: Agricultural drainage ditches are vital for many agricultural landscapes in the U.S. Previous research has indicated that dredging agricultural drainage ditches may degrade water quality. In this study, we monitored nutrient transport in two drainage ditches for six years (2003-2008), during which two dredging activities occurred. Ditch reach nutrient loads were calculated on a monthly and annual basis for the two ditches, as hydrology and water chemistry were monitored daily during the growing season. When dredging activities occurred within the previous 12 months, reach loads were significantly reduced for all nutrients monitored, with net losses in the dredged reaches of NH4-N (-94 kg), soluble P (SP; -6.6 kg), and total P (TP; -5.4 kg). When examining annual reach loads, the nutrient losses from recently dredged reaches were generally significantly lower than the other reaches during the same year. The apparent improvements in water chemistry seem to be contrary to earlier reports of potentially degraded water quality immediately after dredging. We attribute this to: (1) oxidation of reduced sediments, (2) deposition of “fresh” sediments, (3) recolonization by filamentous algae and higher plants, and (4) formation of biofilms on the exposed sediments. To avoid the detrimental impacts on water quality immediately after dredging, and to maximize the benefit of ditch recovery, we propose that ditch managers work with agricultural producers to delay nutrient applications to adjacent fields for at least one month after dredging activities. This should allow the ditches sufficient time to recover their ecological function following dredging.