Synoptic Diagnosis of Catchment Hydrologic and Biogeochemical Processes Using Concentration-Discharge Relationships in the Great Lakes Basin

Abstract:

Hydrologic and biogeochemical processes are strongly coupled and their interactions at the catchment scale can be deciphered by concentration-discharge (C-Q) relationships in streamflow. A weak C-Q power-law relation, as expressed by C = aQ^b (where a and b are constants), indicates chemostasis. A strong C-Q power-law relation is chemodynamic (mixing) as a result of dilution (b < 0) or enrichment (b > 0) of chemical concentrations with a significant increase in streamflow, suggesting differences in chemical composition between shallower/quicker and deeper/slower flowpaths. C-Q relationships of up to 98 catchments from 1951 to 2019 were examined for the Great Lakes Basin, with data from the Water Quality Portal. The results demonstrated that specific conductance, Ca²⁺, Mg²⁺, Na⁺, and Sr behaved similarly in >90% of the catchments, with a strong dilution (mean r² = 0.32 – 0.54, p < 0.01; mean b = -0.25 - -0.14) from rainwater and snowmelt. Cl^- and SO₄^2- were similar, but had lower r² (mean = 0.27-0.29). This suggests that these solutes were primarily controlled by mixing of several end-members with much lower concentrations in shallower/quicker compared to deeper/slower flowpaths. Si was mostly chemostatic over all the catchments. Si concentrations are usually neglectable in rainwater and snowmelt but very high in groundwater, suggesting that overland flow was not a significant end-member. C-Q of NO₃^- included chemostatic (~1/3 catchments), strong chemodynamic (b > 0 & < 0), and weak chemodynamic (r² < 0.1 but p < 0.05; ~ a quarter) patterns. This suggests that the distribution of NO₃^- from top soil to deep regolith varies over catchments. Chemodynamic catchments with b < 0 may have deeper groundwater contaminated by NO₃^-, while those with b > 0 have NO₃^- primarily stored in shallow soils. P showed very weak chemodynamic behavior in most catchments with b > 0, suggesting P in streamflow was mainly from shallow soils. K⁺, NO₂^-, NH₄⁺, Fe and Mn were often chemostastic, resulting from chemical reactions or invariant concentrations over all end-members. Though very simple, C-Q patterns across a combination of different types of solutes could be powerful in examining catchment hydrologic and biogeochemical processes and gain insights into the vertical distribution of chemical compositions in the Critical Zone.