Characterizing Agricultural Impacts on Surface Water Quality

This study was conducted in the agriculturally dominated South Nation River basin located in Eastern Ontario, Canada. The basin has a catchment area of approximately 3900 km² (Fig. 1A). Six water sampling sites were selected in 2016 (Site: S5, S6, S10, S18, S20, and S24) and an additional three were added (S19, S21, S253) in 2017 and 2018 (Fig. 1, Table 1). These study sites have been described in detail previously [11 (link), 20 (link)]. Dairy livestock farming is a dominant land use in the basin, and liquid and solid livestock manure is frequently applied to land as fertilizer in spring and fall. Corn, soybeans, and grass or alfalfa forage are the predominant crops grown.Fig. 1

Study sites. Location of South Nation river basin in eastern Ontario (A); Sample locations in the river basin (B); (C) Land use features of the nine stream sites: S18, S19, S20, and S21 are agricultural drainage ditch sites, S253 (denoted as Agri_253) is located on a larger stream (Strahler stream order = 4), S5, S6, and S10 are under mixed agricultural and urban development, while S24 is in a forested area under no known influence of anthropogenic activity. Maps sources: Google Maps, OpenStreetMap, and Conservation Ontario (https://camaps.maps.arcgis.com/home/index.html); (D) Partial least squares discriminant analysis (PLS-DA) for stream site classification using land use, hydrological features, and water physiochemical properties. The detailed descriptions of the water physicochemical properties can be found in Table 2

Table 1

Sample site descriptions. Land uses defined for 200 m radius around each sample site

SITE ID	Land use type	Upstream contributing area(km2)	Strahler stream order	Water(% land)	Urban/Developed(% land)	Forest(% land)	Wetland(% land)	Agriculture(% land)	Other(% land)
S5	Mixed	81	4	0	7.79	71.46	1.93	16.53	2.29
S6	Mixed	176	5	0	8.42	31.14	0	60.44	0
S10	Mixed	67.5	4	23.77	10.34	2.91	3.63	57.33	2.01
S18	Agriculture	< 5	2	0	0.63	0	0	99.37	0
S19	Agriculture	< 5	2	0	3.7	0	0	96.3	0
S20	Agriculture	< 5	2	0	0	0	0	100	0
S21	Agriculture	< 5	2	0	13.33	0	0	86.67	0
S253	Agriculture	51	4	0	7.57	0	0	92.43	0
S24	Forest	< 5	1	0	6.31	93.69	0	0	0

Sampling sites are located on surface water catchments ranging from < 5 to ∼180 km² (Table 1). S18, S19, S20 and S21 are located on agricultural drainage ditches (Strahler stream orders ≤2) fed almost exclusively by agricultural sub-surface tile drainage [33 (link)]. S253 (denoted as Agri_253) is located on a larger stream (Strahler stream order 4) fed primarily by agriculturally impacted waters. S5, S6, and S10 are intermediate tributaries (Strahler stream order 4–5) that feed the main river directly and are influenced by a mix of agricultural and urban activities. S24 is located on a small stream (Strahler stream order 1). It drains a forested-wetland area not impacted by any known anthropogenic land use activity. S24 serves as a proxy reference site in the context of anthropogenic land use activity in the study [12 (link)]. Only one reference site was included in our study due to accessibility and availability in a region where most watersheds are impacted in some way by anthropogenic activity. We sampled all sites that were logistically possible within the same day in order to process the samples within 24 hours.
Land use upstream of each site was characterized using the methods described by Wilkes et al. [11 (link)] and was classified coarsely into agricultural land, urban/developed land, treed land, wetlands, water, or other (Table 1). Land use data were obtained in the form of raster layers from Agriculture and Agri-Food Canada (AAFC)‘s Annual Crop Inventory [34 ]. Surface water catchment areas (entire catchment upstream of sample site, and catchment area associated with maximum stream length of 5 km upstream of a sample site) and flow direction were determined within ArcMap 9.2 (Environmental Systems Research Institute, Redlands, CA). Stream order was determined using methods described by Lyautey et al. [35 (link)]. Daily air temperature and rainfall were measured from a Hobo weather station (Onset Computer Corporation, Bourne, MA) near S20. Cumulative rainfall was calculated for the day of sampling and 1, 2, 3, 5, 7, and 10 days prior to sampling.