Nitrate Trends in Minnesota Rivers

Regular sampling of river and stream water for nitrate began at numerous sites on Minnesota's rivers during the mid-1970s, and many of these sites continued to be monitored through 2008-2011. A few of these sites were previously assessed for nitrogen (N) load and concentration temporal trends, as is reported in Chapter C2. However, most sites have either not been assessed for nitrate trends or have been studied for trends using a shorter period of time and different statistical methods compared to this study. The objective of this study was to assess long-term trends (30 to 35 years) of flow-adjusted concentrations of nitrite+nitrate-N (hereinafter referred to as nitrate) in a way that would allow us to discern changing trends. Recognizing that these trends are commonly different from one river to another river and from one part of the state to another, our objective was to examine as many river monitoring sites across the state as possible for which sufficient long term streamflow and concentration data were available. The nitrate concentration parameter was chosen for trend analyses for the following reasons: · Nitrate is the dominant form of N in most streams with elevated total nitrogen (TN) concentrations (see Chapter B2). · Nitrate can have adverse human and aquatic-life impacts at high concentrations (see Chapter A2). · Nitrate concentrations in Minnesota rivers and streams are mostly elevated as a result of human activities (see Chapter A2). · The ammonia+ammonium form of N has been consistently shown in previous studies to have decreased substantially since the late 1970s (see Chapter C2), and no additional trend analysis of that N parameter was considered to be needed at this time. · Fewer long-term data are available for TN as compared to nitrate. Nitrate concentration trend analyses can be used to help us understand how human activities and other factors have affected stream nitrate over different time periods. One challenge, however when interpreting nitrate trend results, is a lag time that occurs between changes to the land and the corresponding change to stream N concentrations, especially where slow moving groundwater is a dominant contributor to streamflow and nitrate loads. In some areas, it can take many years for groundwater to move into surface water. In areas other areas where groundwater flow to streams is much quicker, such as tile-drained lands and karst lands, the land changes can affect stream water quality within a much shorter period of time. Nitrate load trends were not assessed in this study because the monitoring frequency at most sites was insufficient for load-trend analyses, and most of the sites where load trends could be determined were already reported by Lafrancois et al. (2013) for the 1976-2005 time period (see Chapter C2).