SEDIMENT RECENCY IN THE WESTERN LAKE ERIE BASIN

Phosphorus (P) cycling in the Western Lake Erie Basin (WLEB) as a result of agricultural runoff has long been known to promote harmful algal blooms, yet there is a lack of data on sediment-bound phosphorus (sedP) recency and origin. Fallout radionuclides ⁷Be and excess ²¹⁰Pb (²¹⁰Pb_ex) can fill important knowledge gaps related to the emergence of sedP in WLEB tributaries.

We measured ⁷Be and ²¹⁰Pbex in baseflow sediment samples from mainstem (Auglaize River and Maumee River) and headwater (Little Flatrock Creek and Black Creek) tributaries collected in 8-9/21, and 9/22. We also analyzed ⁷Be and ²¹⁰Pb_ex in suspended sediment collected during 3 storm events in Black Creek 3/22, 5/22, and 7/22.

In our preliminary data, suspended sediment from the storm samples had higher concentrations of ⁷Be than bed sediments, which tended to have low ⁷Be activity and low but present ²¹⁰Pb activity. March storm sediments had much higher ⁷Be values than those measured during other storms while July storm sediments had much lower ²¹⁰Pb values.

⁷Be and ²¹⁰Pb_ex concentrations in bed sediment suggest it has been in the river for relatively long periods of time. These sediments had low ⁷Be (T1/2 = 53 days), implying sediment was last exposed to direct rainfall hundreds of days ago. The presence, albeit with low activity, of ²¹⁰Pb_ex suggests sediment was decades old, rather than centuries. Bed sediment from 9/2022 had higher ⁷Be values than sediment collected in 8-9/2021. This could be due to extremely low discharges in 2022 leading to wetted channel area contraction and thus direct dosing of channel bed sediment by infrequent rain events.

Storm sediment samples had the highest levels of ⁷Be, suggesting that sediment was recently exposed to rainfall. In samples where ⁷Be was present but ²¹⁰Pb_ex was not, we hypothesize that sediment was sourced from deeply eroded horizontal surfaces, possibly the pasture and cropland sources identified in previous studies. Significantly more rain during the March storm was correlated with the highest ⁷Be levels.

Other parts of this project will determine sedP levels and sediment origin to provide a thorough understanding of P loading in the watershed. Given these data, this research will further differentiate P in the stream from upstream runoff from agricultural fields relative to re-mobilization of in-channel storage.