HYDROCHEMICAL RESPONSE OF AVERY BROOK TO A FALL RAIN EVENT: THE ROLE OF FLOWPATH

Increases in total rainfall, rainfall intensity, and soil temperature associated with changing climate in the northeastern US may lead to increases in dissolved organic carbon (DOC) and mercury in streamwater draining forested headwater catchments. Avery Brook, located in West Whately, MA, lies within a 756 ha forested catchment and is the principal tributary to the water supply reservoir for the nearby city of Northampton. When DOC from the stream reacts with chlorine introduced during water treatment, carcinogenic disinfection byproducts are produced.

Hourly water samples were collected at the gage station on Avery Brook during an intense rain event (63mm) on October 28^th, 2015 using an ISCO autosampler. In response to the rain, discharge increased from 3 cfs to 110 cfs at peak flow and specific conductance dropped from 30µS to 22µS/cm then recovered to 26µS/cm. Water samples collected during the event were filtered and analyzed for DOC, Hg (AF), alkalinity, UV₂₅₄, and major cations (ICP OES) and anions (IC).

Both DOC and UV₂₅₄ increased with the stream discharge while most other constituents decreased due to dilution. The greater than threefold increase in DOC (3 to 10mg/L) reflects flow of water through organic rich soil horizons and riparian wetlands. There was a similar increase in UV₂₅₄ (0.10 – 0.35) but the Specific Ultraviolet Absorption (SUVA) only increased from 2.3 to 3.3. While both the DOC and UV₂₅₄, values rose and fell with stream discharge, the SUVA values rose with discharge to the peak value and plateaued. Higher SUVA values indicate an increase in the aromaticity of the DOC, suggesting that stream Hg concentrations are also increasing.

Under pre-storm baseflow conditions stream flow is supported almost exclusively by groundwater. Increasing flow during the event comes primarily from water moving along shallow flow paths through soil organic horizons in areas of thin till or steep slopes. Riparian wetlands are also important in rapidly supplying high DOC water to the stream during rain events. As stream flow decreases after peak flow groundwater and interflow through deeper mineral horizons becomes relatively more important until the stream returns to baseflow conditions.