Measurements of pools and fluxes of transport or cycling of elements and compounds through the biotic and abiotic components provide critical information about the function of ecosystems. Because the time for a molecule to be completely transported through an ecosystem may be decades to millennia, long-term data provide one of the few means to estimate how ecosystems use and respond to changes in inputs of nutrients and toxic substances. This chapter tests two hypotheses related to patterns in biogeochemistry across sites: (1) patterns in atmospheric deposition over the past 20 years are different for the eastern and western parts of the U.S., and (2) changes in atmospheric deposition are related to changes in human population density for some sites. Long-term data, collected at multiple sites across the US, in chemical measurements in wet deposition, e.g., sulfur dioxide, nitrogen oxide, and ammonia emission data, and in human population size support both hypotheses.  The results illustrate that the effects of human activities on biogeochemistry vary regionally and across the continent, and demonstrate that cross-site comparisons of long-term data can provide new insights into the spatial patterns of transport and cycling of elements and compounds.