Have done some NEPA fishing with alum test kit. As noted, aluminum is a better predictor of trout mortality with acid precip/rain than pH. Bedrock geology is a useful signal of stream aluminum levels that you can get from a map. For example, in Sullivan county, burgoon is a bad sign, and catskill or huntley a better sign, but there can be variation w/in a bedock band due to soil differences (see quote below).
Striking to see the aluminum differences in nearby streams that look the same. I checked two parallel tribs, .5 mi apart, to one small Sullivan county stream at moderate flows. The first trib had .15 alum (3/4 of the lethal level) and burgoon bedrock: won't even be brookies at that level. The second trib was in huntley, had no alum at all, and had both brook and brown trout.
Interestingly, alum levels associated with bedrock and soils will sometimes lead to worse fishing as you climb higher up a mountain -- the streams up high may start or stay in burgoon or Pottsville. You may be hiking into areas with higher gradient, better pools, lower fishing pressure, and less fish. Not that I have ever done it 🙂
"The general pattern observed for the watershed is that higher degrees of acidification tend to correlate with higher elevations, corresponding to the Burgoon Sandstone. The lowermost reaches floored by the Catskill Formation show the best water quality. The better conditions in the lower main stem of East Branch Fishing Creek are also associated with the less acidic Barbour soil series. There is insufficient overall correlation to definitively point to any one geologic unit as being the source of the acidification problems. Because of the consistently strong aluminum response to flow throughout the watershed, it is interpreted that runoff acidification is primarily related to systemic soil acidification and not a particular bedrock component."
http://www.wehydro.com/images/2007fc.pdf
somewhat blurry images, but some nice bedrock maps here:
https://www.libraries.psu.edu/psul/emsl/paresources/paprelimquadmaps.html