Land use and hydrologic condition: Effects on water quality in southern Ontario streams

Over the past 40 years, agricultural land cover has been declining and urban cover has become the dominant land use in Ontario, with the majority of this expansion occurring in southern Ontario. This shift in land use could impact both stream water quality and runoff. This thesis took two approaches to investigate this impact. First, analysis of long-term (up to 35 yrs) total phosphorus (TP) and chloride (Cl) data for 113 streams across southern Ontario revealed two patterns in water quality: TP decreased and Cl increased at 61/113 and TP was stable/insignificant and Cl increased at 33/113 streams. Many of these streams (49) have or once had a sewage treatment plant upstream of the monitoring station, and improvements in P removal at these stations may account for declines in TP. At the other 64 streams, however, a very similar pattern was observed with TP declines and increases in Cl at 2/3 of the stations and stable/increasing TP and increasing Cl at the other 1/3. Land use and population data for this region suggest that replacement of agricultural land by urban landscapes may be contributing to declines in TP and coincident increases in Cl. Part II of this thesis involved high-intensity monitoring of two adjacent catchments with primarily agricultural land use (> 63% agriculture), one of which is undergoing urbanization. The average Cl concentrations at the 'urbanizing' Jennings Creek catchment (17.5% urban) was 7-times greater than at the neighbouring McLarens Creek catchment, which confirms the influence of urban land cover on stream Cl. However, TP, NO 3-N and DOC concentrations and export were relatively similar between the two catchments and likewise, stream flow (total runoff, peak flow, event length) was comparable at the two streams, perhaps because of the similar and large proportion of agricultural land cover at the two catchments. Storm events accounted for the bulk of TP export at both sites, primarily due to increases in particulate P during high runoff events. In contrast, stream N was present primarily in the dissolved inorganic form (NO3-N, nitrate) and total organic carbon was also predominantly in the dissolved form (DOC). In contrast to TP, N and C export were not sensitive to storm events. Overall, these results indicate that land use has a strong influence on water quality, although the hypothesized impact of urban land cover on stream flow was not detected. Chloride levels are expected to continue to rise as urban land cover replaces agriculture in southern Ontario, whereas TP levels may decline due to replacement of relatively high TP agricultural land with impervious surfaces.