The effects of a whole-watershed calcium addition on the chemistry of stream storm events at the Hubbard Brook Experimental Forest in NH, USA

Patterns of storm runoff chemistry from a wollastonite (calcium-silicate mineral, CaSiO₃) treated watershed (W1) were compared with a reference watershed (W6) at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire (NH), USA to investigate the role of Ca²⁺ supply in the acid-base status of stream chemistry. In the summer of 2003, six storm events were studied in W1 and W6 to evaluate the effects of the wollastonite treatment on the episodic acidification of stream waters. Although mean values of Ca²⁺ concentrations decreased slightly from 33.8 to 31.7 μmol/L with increasing stream discharge in W1 during the events, the mean value of acid neutralizing capacity (ANC) was positive (1.2 μeq/L) during storm events, compared to negative values (- 0.2 μeq/L) in W6. This pattern is presumably due to enhanced Ca²⁺ supply in W1 (20.7 to 29.0% of dissolved Ca²⁺ derived from the added wollastonite) to stream water as a result of interflow along shallow flowpaths. In addition, the application of wollastonite increased pH and dissolved silica (H₄SiO₄) concentrations, and decreased the concentration of inorganic monomeric Al (Al_i) in W1 in comparison with W6 during storm events. Despite an increase in SO₄^2- concentration, likely due to desorption of sulfate from soil after the treatment, the watershed showed an increase in ANC compared to the reference watershed, serving to mitigate episodic acidification.