Urbanization drives geographically heterogeneous freshwater salinization in the northeastern United States

Rising trends in freshwater salinity, collectively termed the Freshwater
Salinization Syndrome (FSS), constitute a global environmental concern.
Given that the FSS has been observed in diverse settings, key questions
regarding the causes, trend magnitudes, and consequences remain. Prior
work hypothesized that FSS is driven by state factors, such as
human-centered land use change, geology, and climate. Here, we identify
the fundamental overriding factors driving FSS within the northeastern
United States and quantify the diversity of FSS severity within the
region. Specifically, we analyzed decadal-scale trends in specific
conductance (a salinity proxy) for 333 lotic sites over four 
decades. Next, we quantified potential variables driving the rising or
falling trends, including impervious surface cover (ISC), winter
temperature and precipitation, watershed size, and ambient conductance.
Temperature and ISC were considered the most likely candidates for
predicting FSS severity because road salts have previously emerged as the
fundamental regional driver.Most (62.5%) sites exhibited patterns of
significantly increasing conductance; thus, the overall regional state
reflects advancing FSS. However, others exhibited an absence of change
(28.8%) or decreasing values (8.7%), and slope magnitude did change with
latitude. Linear modeling demonstrated that two variables—ISC and
watershed size—constitute the best predictors of long-termconductance
trends and that an intercept not significantly different than zero
suggests that the FSS does not reign in the absence of urbanization. We
also detected areas with consistently decreasing trends despite moderate
ISC. Therefore, within the region, advancing urbanization causes the
typical condition of advancing FSS, but heterogeneity also exists.