Ammonia toxicity: Impact assessment of combined sewer overflows on the River Spree in Berlin

Ammonia toxicity: Impact assessment of combined sewer overflows on the River Spree in Berlin

Combined sewer overflows can lead to acute, critical conditions for aquatic organisms in receiving surface waters (Borchardt et al. 2007; FWR 1998; Harremoes et al. 1996; Krejci et al. 2004; Lammersen 1997). Based on the river type of the River Spree, CSO impacts of possible concern were identified to be high ammonia (NH3) and low dissolved oxygen concentrations (DO) (Senatsverwaltung für Stadtentwicklung 2001; Leszinski et al. 2007). For DO, existing continuous measurements from the River Spree from 2000 to 2007 were assessed in detail in the KWB report by Riechel (2009). However, Riechel (2009) neglected NH3 toxicity assessment, since no continuous NH3 measurements were available. The present report aims at filling this gap by estimating the potential for toxic NH3 concentrations in the River Spree with recent data. Based on stormwater impact guidelines for ammonia, critical total ammonium concentrations ([NH4,tot] = [NH4+] + [NH3]) were calculated and compared to continuous NH4,tot measurements in the Berlin River Spree. NH4,tot was measured i) at a heavily CSO impacted river stretch (year 2011) and ii) at a monitoring station several kilometres downstream of the combined sewer area (years 2010 and 2011). The analysis led to the following results: (i) Two years of continuous NH4,tot measurements showed clear increases in NH4,tot due to CSO but no occurrence of critical toxicity levels for cyprinid fish, according to Lammersen (1997) (ii) Maximal observed concentration of ~1.3 mg-N-NH4,tot l-1 was ~5 times smaller than the lowest existing threshold, which would need to be exceeded for 24 h to be considered as critical. The observed maximal concentration peak had a duration of only 3 h. The threshold, corresponding to the 3 h-duration would be even ~8 times higher than the observed ~1.3 mg-N-NH4,tot l-1. (iii) Ammonia toxicity would only be possible if maximal NH4,tot occurred during highest sensitivity of the river due to very high pH > 9. However, it was observed that pH drops significantly during CSO impacts due to low pH in rain water, which makes pH > 9 during CSO very unlikely. Given the results, the risk for ammonia toxicity due to CSO is judged as very low, particularly in comparison with regular problematic DO conditions after CSO events in summer.