Nature and science
McDonalds Branch: acid rain meets an already acidic stream
Documented history and research · Sources compared
Decades of water samples made a small forest watershed a test of both environmental change and the methods used to measure it.
Following water through the forest
A USGS acidic-deposition investigation during 1983–1986 measured more than stream water. Researchers collected precipitation, water passing through the canopy, soil water and groundwater, alongside soils and hydrological observations. This let them examine changes along the water’s route instead of assuming rain chemistry remained unchanged after entering the forest. Lord and colleagues, hydrogeochemical data (1990).
The report gives a mean precipitation pH of about 4.3 for January 1985 through March 1986. Stream measurements ranged from 3.2 to 4.8 and generally became less acidic downstream. Aluminum concentrations were elevated at some locations. These are observations from a specified study period, not a present-day description of every Pinelands stream. Lord and colleagues, hydrogeochemical data (1990).
The measuring method became part of the evidence
A later USGS benchmark-basin assessment analyzed 247 samples collected from 1963 through 1995. It discussed changes in laboratories, sampling frequency and analytical methods. In particular, a sulfate method used during part of the record could overestimate concentrations in dilute, strongly colored water. A change in a chart could therefore reflect a measurement change as well as an environmental one. USGS benchmark-basin assessment (1999).
The assessment also noted an earlier follow-up study that found no significant long-term pH decline. That does not establish that acidic deposition had no effects. It shows why the history cannot be reduced to a naturally clean stream becoming acidic: investigators had to distinguish the basin’s existing chemistry, atmospheric inputs and the behavior of their instruments. USGS benchmark-basin assessment (1999).
Sources and research limits
- Lord and colleagues, hydrogeochemical data (1990) — Complete institutional abstract inspected.
- USGS benchmark-basin assessment (1999) — Indexed historical-data and analytical-method discussion inspected; direct retrieval failed.
Original laboratory sheets and the separately cited pH-trend study were not read. The later assessment’s discussion is attributed rather than represented as a new analysis here.