Regenerative stormwater conveyances: Novel ecosystems exhibiting iron flocculate formation may be altering metal biogeochemical cycles

Background/Question/Methods

One ecological engineering design to reduce stormwater runoff and pollutants is called a Regenerative Stormwater Conveyance (RSC). For about a decade, RSCs have been implemented in degraded headwater streams using sand, woodchips, and stone to create stepped-pool structures in which the majority of flow occurs as groundwater through a thick seepage bed. RSCs have been effective at reducing the flashiness of stormwater runoff, thereby preventing streambed erosion; however, little was known about the ecological consequences associated with implementing these structures. In particular, the accumulation of iron flocculate associated with iron-oxidizing bacteria has been observed at RSC sites, yet it was unknown whether the accumulation of this flocculate was predominately natural, a consequence of leached iron (Fe) from the materials used in the RSC construction, or because of mobilized Fe associated with higher groundwater levels in adjacent riparian zones that occur after construction. Water samples were collected from riparian wells, hyporheic wells, and surface water at 6 RSCs and 3 reference streams through the Piedmont and Coastal Plain physiographic provences; samples were analyzed for metals, nutrients, and DOC. Iron-flocculate was collected from quadrats at these sites and physical habitat parameters were evaluated using Maryland Biological Stream Survey methods. 

Results/Conclusions

The RSCs studied had higher surface-water Fe concentrations than reference streams in the Piedmont physiographic province, as well as increased OM/Fe ratios in bacterial mats relative to reference streams. Iron bacterial mat growth seemed to be contributed to by the use of ironstone boulders in RSC construction, as well as riparian soil-sourced Fe. The major contributing factor to Fe solubilization, and thus availability to Fe-oxidizing bacteria, was the availability of DOC to Fe-reducing bacteria in the presence of Fe containing substrates; these conditions had been created in the RSCs. While RSCs improve many physical habitat parameters when compared to urban streams, accumulations of Fe flocculate and the pool structure itself may preclude use by many organisms, even though some clearly thrive there and create unusual bacterial mat communities at some sites. Compared to degraded urban streams RSCs may be an improvement, though questions remain about the long-term effectiveness and effects of their implementation.