Quantifying the influence of bedrock groundwater discharge on spatial variation in stream C, N, and P concentrations in forest headwaters

Groundwater discharge through bedrock into streams impacts carbon (C), nitrogen (N), and phosphorous (P) concentrations in streamwater. However, insufficient quantitative evaluation of the effects of bedrock groundwater discharge has been performed to date. This study quantified the effects of bedrock groundwater discharge on spatial variation in C, N, and P concentrations in streamwater.

We targeted a forest headwater catchment with an area of 5 km² in Japan. We observed streamwater at 68 and 76 points, including several bedrock springs, in September and December 2013, respectively. Groundwater in the soil layer was also sampled on a hillslope within the catchment. We analyzed concentrations of major ions (Na⁺, NH₄⁺, Mg²⁺, Ca²⁺, Cl^-, NO₃^-, and SO₄^2-), dissolved organic C (DOC), and dissolved inorganic P (DIP). Dissolved inorganic N (DIN) concentration was defined as the sum of those of NH₄⁺ and NO₃^-.

We hypothesized that streamwater consisted of a mixture of groundwater discharged through soil and bedrock, and that spatial variation in streamwater chemistry was due to differences in their mixing ratios. We calculated the mixing ratios of each groundwater from streamwater Na⁺, Mg²⁺, Ca²⁺, Cl^-, and SO₄^2- concentrations and then predicted streamwater DOC, DIN, and DIP concentrations by multiplying these mixing ratios with groundwater DOC, DIN, and DIP concentrations and calculating the sum.

DOC concentrations observed in September and DIP concentrations observed in both September and December were positively linearly correlated with predicted values (r² = 0.44, 0.80, and 0.49, respectively; p < 0.001 for all). Predicted DOC and DIP concentrations underestimated and overestimated the observed values, respectively. These results highlight the importance of bedrock groundwater discharge in forming streamwater DOC and DIP concentrations and also demonstrate that models considering only groundwater discharged through soil and bedrock cannot not fully predict spatial variation in streamwater DOC and DIP concentration values. Differences between observed and predicted values may be caused by within-stream DOC production and DIP absorption and/or spatial heterogeneity in groundwater chemistry. Future studies should clarify the factors that cause these differences, to achieve better predictions.