A tale of two contaminants: stark differences in the response of N and P to urban lake mitigation efforts

A constructed in-lake water quality mitigation system has proven itself to be effective at reducing Machado Lake phosphorus (P) levels, but ineffective at reducing nitrogen (N) levels. A combination of lake sediment dredging and capping, oxygenation, and a recirculating wetland have reduced lake water column P levels by nearly 50%, as compared to pre-project levels. Key to this result has been the dampening of seasonal P recycling in the sediments. A new lake water quality numerical model is presented, with applications to both pre- and post-project conditions. Model auditing has revealed very good results with respect to predicting mitigation impacts on P but poor results with respect to predicting the performance, or lack thereof, of the N mitigation system. Model sensitivity analyses indicate that the P reductions are primarily attributable to the sediment dredging and capping. Conversely, seasonal data, supported by modeling, suggest that the poor performance of the N mitigation system may be attributable to incomplete removal, or sequestration, of sediment N mass during dredging and/or a lack of impact from the oxygenation system. Future mitigation efforts for the lake should focus on reducing the substantial watershed nutrient loads to the lake and further in-lake P inactivation.