Modeling future land cover and water quality change in Minneapolis, MN, USA to support drinking water source protection decisions
We developed 2011-2050 land cover change scenarios and modeled the impact of projected land cover change on influent water quality to support long-term planning for the Minneapolis, MN Water Treatment Distribution Service (MWTDS). Projected land cover changes based on relatively unconstrained growth (2 scenarios) led to substantial increases in nitrogen (TN) loads, modest increases in phosphorus (TP) loads, and substantial declines in sediment loads during the spring. Changes in sediment, TN, and TP during summer, fall, and winter and for all seasons for two “constrained” growth scenarios were near zero (no change) or declined modestly. Longitudinal analysis suggested that the Mississippi River corridor (and its extant vegetation) immediately upstream of MWTDS is a sediment (and phosphorus) “trap.” For the unconstrained growth scenarios, projected spring season median sediment loads increased substantially at a site immediately upstream of MWTDS, whereas projected spring season sediment loads at MWTDS for the same scenarios decreased substantially. Autoregressive analysis of current (2008-2017) chemical treatment application rates (mass/water volume processed) and extant (2001-2011) land cover change were consistent with existing literature in that statistically significant increases in chemical treatment application rates were temporally congruent with increases in urban land cover and conversion of pasture to cropland. Using the current trend in chemical treatment application rates and their inferred relationship to extant land cover change as a bellwether, our projections for the unconstrained growth scenarios suggest that future land cover change will present challenges to the production of potable water.