Could a reported rainfall pH of 3.1 actually be realistic?
A quick literature search seems to confirm Gordons estimation, even at the scale of a whole bucket:


  [T]he annual mean pH, based upon
  samples collected weekly during 1970-1971 and weighted proportionally to
  the amount of water and pH during each period of precipitation, was 4.03
  at the Hubbard Brook Experimental Forest, New Hampshire; 3.98 at Ithaca,
  New York; 3.91 at Aurora, New York; and 4.02 at Geneva, New York. Measurements
  on individual rainstorms frequently showed values between pH 3
  and 4 at all of these locations. Data from the National Center for Atmospheric
  Research included precipitation pH values as low as 2.1 in the northeastern
  United States during November 1964. 


Patel, C. K. N., E. G. Burkhardt, and C. A. Lambert. "Acid rain: a serious regional environmental problem." Science 184 (1974): 1176-1179.