Determining the elusive baseline for surface water chemistry at the Hubbard Brook Experimental Forest

Background/Question/Methods

Knowledge of baseline conditions is important for quantitative evaluation of the effect of human activities on environmental conditions, such as the impact of acid deposition.  Efforts to restore ecosystems to prior, “pristine” condition require restoration targets, often based on some presumed baseline condition.  In spite of the importance of knowing about baseline conditions, they are often unknown or exceedingly difficult to determine. 

Results/Conclusions

Rapid and relentless dilution of surface water chemistry is occurring in the White Mountains of New Hampshire, following decades of acid deposition.  Extrapolating measured linear trends (with an r² >0.84 since 1985) for a uniquely long data set of up to 47 years, suggest that both precipitation and streamwater chemistry in the Hubbard Brook Experimental Forest will approximate distilled water within one to three decades.  Because such dilute chemistry is unrealistic for surface waters, theoretical baseline compositions for precipitation and streamwater chemistry have been calculated: electrical conductivity of 3 and 5 µS/cm, base cation  concentrations of  7 and 39 µeq/liter, acid-neutralizing capacity values of <1 and 14 µeq/liter, respectively; and pH of 5.5 for both precipitation and stream water.  Rapid dilution of surface waters has important ecological, biogeochemical and resource management implications.