Concern regarding the impacts of continued nitrogen and sulfur deposition on ecosystem health has prompted the development of critical acid load assessments for forest soils. A critical acid load is a quantitative estimate of exposure to one or more pollutants at or above which harmful acidification-related effects on sensitive elements of the environment occur. A pollutant load in excess of a critical acid load is termed exceedance. This study combined a simple mass balance equation with national-scale databases to estimate critical acid load and exceedance for forest soils at a 1-km2 spatial resolution across the conterminous US. This study estimated that about 15% of US forest soils are in exceedance of their critical acid load by more than 250 eq ha-1 yr-1, including much of New England and West Virginia. Very few areas of exceedance were predicted in the western US. This simple mass balance equation estimated that 17% of US forest soils exceed their critical acid load by more than 250 eq ha-1 yr-1, and these areas are predominantly located in the northeastern US.

Simple mass balance equations (SMBE) of critical acid loads (CAL) in forest soil were developed to assess potential risks of air pollutants to ecosystems. However, to apply SMBE reliably at large scales, SMBE must be tested for adequacy and uncertainty. Our goal was to provide a detailed analysis of uncertainty in SMBE so that sound strategies for scaling up CAL estimates to the national scale could be developed. Specifically, we wanted to quantify CAL uncertainty under natural variability in 17 model parameters, and determine their relative contributions in predicting CAL. Results indicated that uncertainty in CAL came primarily from components of base cation weathering (BCw; 49%) and acid neutralizing capacity (46%), whereas the most critical parameters were BCw base rate (62%), soil depth (20%), and soil temperature (11%). Thus, improvements in estimates of these factors are crucial to reducing uncertainty and successfully scaling up SMBE for national assessments of CAL.

For the routine determination of metals in environmental samples, we require microwave-assisted digestion methods that yield 'total' or 'near-total' recoveries while avoiding the use of HF acid. As inductively coupled plasma mass spectrometry (ICP-MS) is the method of detection, it is desirable to minimize the use of HCl to avoid spectral interferences caused by high Cl- concentrations. Using certified reference materials, we performed a series of modifications to the US EPA method 3051 which included: increasing the temperature and durations of microwave digestion, varying the ratio of sample mass to acid volume, and alterations to the compositions of the acid digestion mixture. The experiments were conducted using urban particulate matter (NIST-1648), coal fly ash (NBS-1633) and six CANMET certified reference materials (Till-2, Till-3, Till-4, LKSD-1, LKSD-2 and LKSD-4), in two laboratories (Health Canada and Environment Canada) using different microwave digestion systems and different ICP-MS instruments. Our modified microwave-assisted nitric acid digestion method improved recoveries for Pb, Zn, V, Fe and Cu approaching 'total' recoveries in the same matrices determined using X-ray fluorescence (XRF) and instrumental neutron activation analysis (INAA) as reported in the certificates of analysis. Recoveries for other elements such as Cr and Ni compared well with 'near-total' recoveries yielded by traditional (non-assisted) acid digestion methods.