Surface water quality is improving due to declining atmospheric N deposition.

We evaluated long-term surface water nitrate and atmospheric nitrogen (N) deposition trends for a group of nine predominantly forested Appalachian Mountain watersheds during a recent multidecadal period (1986-2009) in which regional NOx emissions have been progressively reduced. Statistical analysis showed unexpected linear declines in both annual surface water nitrate-N concentrations (mean =46.4%) and yields (mean =47.7%) among the watersheds corresponding to comparable declines in annual wet N deposition (mean =34.4%) resulting from U.S. NOx emission control programs during the same time period. Nitrate-N concentration trends were robust across a large geographical region and appeared insensitive to watershed size across several orders of magnitude-suggesting that the improvements in water quality are probably propagated to surface and estuarine waters downstream. Surface waters are thus responding to declining atmospheric N deposition in much the same way they responded to declining sulfur deposition-although only one watershed showed a 1:1 relationship. Application of a kinetic N saturation model indicated that all nine forested watersheds are exhibiting signs of N saturation as evidenced by a limited, but variable, efficiency of demand for N. Further reductions in N deposition would be expected to produce additional reductions in streamwater N loads.

Estimating reference nutrient criteria for Maryland ecoregions.

Management of stream nutrients is becoming increasingly important in order to protect both water quality and aquatic resources throughout the USA. Using an extensive water quality database from the long-term Maryland Biological Stream Survey (MBSS), we describe nutrient relationships to landscape characteristics as total nitrogen (TN) and total phosphorus (TP) of small-order, non-tidal streams in USEPA L2 and L3 ecoregions in Maryland and by MBSS stream order at the L2 and L3 ecoregion levels. To protect stream ecosystem integrity, preliminary reference nutrient estimates (TN and TP) as percentiles (25th of all stream reaches and 75th of stream reference reaches) for the six Maryland L3 ecoregions are: Blue Ridge TN 0.29 and 0.64 mg/L, TP 0.0065 and 0.0090 mg/L; Central Appalachians TN 0.40 and 1.0 mg/L, TP 0.0060 and 0.015 mg/L; Middle Atlantic Coastal Plains TN 0.93 and 2.5 mg/L, TP 0.094 and 0.065 mg/L; Northern Piedmont TN 1.6 and 1.8 mg/L, TP 0.010 and 0.015 mg/L; Ridge and Valley TN 0.40 and 0.98 mg/L, TP 0.0063 and 0.012 mg/L; and Southeastern Plains TN 0.33 and 0.82 mg/L, TP 0.016 and 0.042 mg/L. High levels of both TN and TP are present in many streams found in non-tidal watersheds associated with all Maryland ecoregions, but are especially elevated in the Northern Piedmont and Middle Atlantic Coastal Plain ecoregions, with the latter second-order streams (average TN > 2.9 mg/L) significantly higher than all other ecoregion-order combinations. Across all six ecoregions, mean nutrient loading for both TN and TP was generally equivalent in first-order streams to nutrient concentrations seen in both second- and third-order streams, indicating a definite need to increase efforts in preventing nutrients from entering first-order streams. Small-order stream nutrient levels are the drivers for subsequent TN and TP inputs into the upper freshwater tidal reaches of the Chesapeake Bay, resulting in a potential risk for altered estuarine ecosystems.

Nutrient concentrations in Maryland non-tidal streams.

Using a spatially extensive database from the Maryland Biological Stream Survey (MBSS), we describe nutrient relationships of small-order, non-tidal streams to Maryland watershed basins, Maryland Tributary Strategy basins, and stream order. In addition, we estimate the number of stream km affected by nutrient loading, using derived nutrient criteria. Based on the MBSS spring water quality sampling, we determined several important factors relating to nutrient levels in non-tidal streams. There are strong linear relationships of nutrients to the percentage of agriculture and forested land present within MBSS sampling strata. Both mean total nitrogen (TN) and mean total phosphorus (TP) levels for watershed basins by stream order show exceedances of derived nutrient reference criteria for Maryland. Four Maryland basins have over 85% of their stream kilometers exceeding the TN criterion, with three basins over 90% of the TP criterion. To protect small stream integrity in Maryland, we recommend an upper stream TN criterion between 1.34 and 1.68 mg/L and an upper stream TP criterion between 0.025 and 0.037 mg/L, based on quantile analyses. Elevated levels of both TN and TP are present in non-tidal streams, with subsequent nutrient inputs into the upper freshwater tidal reaches of the Chesapeake Bay.

Contemporary trends in the acid-base status of two acid-sensitive streams in western Maryland.

Recovery of streamwater acid neutralizing capacity (ANC) resulting from declines in regional acid deposition was examined using contemporary (1990-2005) data from two long-term monitoring stations located on the Appalachian Plateau in western Maryland, U.S. Two computational methods were used to estimate daily, monthly, and annual fluxes and discharge-weighted concentrations of ANC, sulfate, nitrate, and base cations over the period of record, and two statistical methods were used to evaluate long-term trends in fluxes and concentrations. The methods used to estimate concentrations, as well as the statistical techniques, produced very similar results, underlining the robustness of the identified trends. We found clear evidence that streamwater sulfate concentrations have declined at an average rate of about 3 microeq L(-1) yr(-1) at the two sites due to a 34% reduction in wet atmospheric sulfur deposition. Trends in nitrate concentrations appear to be related to other watershed factors, especially forest disturbance. The best evidence of recovery is based on a doubling of ANC (from 21 to 42 microeq L(-1)) at the more acid-sensitive site over the 16-year period. A slowing, or possible reversal, in the sulfate, nitrate, and SBC trends is evident in our data and may portend a decline in the rate of--or end to--further recovery.

Analysis of trends in episodic acidification of streams in western Maryland.

In this study we report on changes in the magnitude and mechanisms of episodic acidification of a small acid-sensitive stream in western Maryland (U.S.) during the 1990s, a period in which wet sulfate deposition declined by 10-25% due to implementation of the Clean Air Act Amendments (CAAA) of 1990. We observed a relatively minor trend in the magnitude of episodic acidification over this period, as measured bytransient changes in acid neutralizing capacity (deltaANC) and minimum values of ANC (ANC(min)) during 22 events sampled prior to and following CAAA implementation. Any relationship to changes in atmospheric deposition appears to be confounded by large hydroclimatological variability between the two sampling periods. Nonetheless, results obtained prior to implementation of the CAAA indicated that the mechanism of episodic acidification was mostly attributable to flushing of accumulated sulfate from the watershed, whereas results obtained post-CAAA indicated domination by base cation dilution. This shift in the mechanism of episodic acidification is qualitatively consistent with hydrochemical theory, as well as with empirical results from surface waters in other regions where dramatic declines in sulfate deposition have taken place.