A Salt (Chloride) Balance for the Minneapolis/St. Paul Metropolitan Area Environment

There are no natural sources of NaCl in the geology of the Twin Cities Metropolitan Area (TCMA) of Minnesota. Salinity in surface water or groundwater of the TCMA must therefore be of anthropogenic origin. The largest salt uses in the TCMA are for water softening and for road de-icing. Water softening occurs in individual households, and in commercial and industrial establishments. Backwash is typically disposed of in sanitary sewers and discharged from wastewater treatment plants to major rivers. Road salt is used for public safety on winter roads. Road salt is a solute in snowmelt water, which runs into storm sewers, small streams, rivers, lakes and wetlands. In the TCMA watershed analyzed, approximately 267,000 short tons (242,000 metric tonnes) of road salt (NaCl) are applied annually for road de-icing. Within the political boundaries of the seven county TCMA, the numbers are 350,000 short tons (315,000 metric tonnes). It is important to understand how these applications influence the environment. Salt balances for the entire TCMA and for 10 smaller sub-watersheds reveal that a large portion of the local road salt applied in the TCMA is not carried away in the Mississippi River water. Streamflow and bi-weekly concentration measurements from 2000 to 2007 indicate that the Minnesota and Mississippi Rivers import 235,000 tonnes of chloride annually from upstream into the Twin Cities Metropolitan Area and 355,000 tonnes are exported by the Mississippi River downstream. 120,000 tonnes are being added to the rivers as they travel through the Twin Cities. Of these 120,000 tonnes, approximately 87,000 tonnes come from wastewater treatment plants as determined from flowrate and biweekly concentration measurements in 2007/2008. The remaining 33,000 tonnes are attributed to road salt, and represents 22% of the total of 148,000 tonnes of chloride that are applied to the roads in the TCMA watershed every year. The latter figure was obtained from a detailed survey and inventory of road salt uses from 2000 to 2006. Roughly, 78% of the road salt applied is staying in the watershed. From chloride balances in 10 small sub-watersheds, slightly lower values of 65 - 73% were calculated.