Nonpoint source urban nutrient loading into streams and receiving water bodies is widely recognized as a major environmental management challenge. A dominant research and management paradigm assumes that loading primarily derives from elevated stormwater. However, baseflow can account for a large portion of total loading, especially for low development intensity watersheds which comprise the largest urban areas. We investigated the sources and drivers of nonpoint source baseflow nitrogen loading across 27 headwater catchments in the urbanized Piedmont region of North Carolina, USA. Nitrate isotopes, predictors of concentration-discharge (CQ) slopes, and predictors of mean annual total dissolved nitrogen (TDN) loading suggest that wastewater was a major baseflow nonpoint source of nitrogen across developed catchments likely contributing 61% of nitrate loading from septic served catchments and 49% from sewer served catchments. Our findings suggest that subsurface TDN was abundant, loading was largely transport limited, and the hydrogeomorphic position of sanitary infrastructure strongly influences transport. We developed an empirical model showing catchment loading increased with the topographic wetness index of sanitary sewer location, convergent sloping land area, parcel density, and residual agricultural landcover (R² = 0.78). We extended this model to the study region's 1,436 developed small (0.3–20.8 sq km) catchments. We estimated up to 92.7% of nonpoint source baseflow TDN loading comes from low and medium development intensity catchments, and sanitary infrastructure in wet areas of the landscape accounts for 39% of regional baseflow loading. Our research indicates that managing baseflow loading will require addressing lower development intensity catchments and sanitary infrastructure.

中文翻译：

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基流氮负荷在发达的城乡梯度上的非点源和传输

非点源城市养分加载到溪流和接收水体中被广泛认为是一项重大的环境管理挑战。一个占主导地位的研究和管理范式假设负荷主要来自高位雨水。然而，基流可以占总负荷的很大一部分，特别是对于包括最大城市地区的低开发强度流域。我们调查了美国北卡罗来纳州皮埃蒙特城市化地区 27 个源头流域的非点源基流氮负荷的来源和驱动因素。硝酸盐同位素，浓度-放电 (CQ) 斜率的预测因子，和平均年总溶解氮 (TDN) 负荷的预测因子表明，废水是已开发集水区的主要基流非点氮源，可能占化粪池服务集水区硝酸盐负荷的 61% 和下水道服务集水区的 49%。我们的研究结果表明，地下 TDN 丰富，负载在很大程度上受到运输限制，并且卫生基础设施的水文地貌位置强烈影响运输。我们开发了一个经验模型，显示集水区负荷随着下水道位置、收敛坡地面积、地块密度和剩余农业用地覆盖的地形湿度指数而增加（卫生基础设施的水文地貌位置对交通有很大影响。我们开发了一个经验模型，显示集水区负荷随着下水道位置、收敛坡地面积、地块密度和剩余农业用地覆盖的地形湿度指数而增加（卫生基础设施的水文地貌位置对交通有很大影响。我们开发了一个经验模型，显示集水区负荷随着下水道位置、收敛坡地面积、地块密度和剩余农业用地覆盖的地形湿度指数而增加（R ²  = 0.78)。我们将此模型扩展到研究区域的 1,436 个已开发的小型（0.3-20.8 平方公里）流域。我们估计，高达 92.7% 的非点源基流 TDN 负荷来自中低开发强度集水区，景观潮湿地区的卫生基础设施占区域基流负荷的 39%。我们的研究表明，管理基流负荷需要解决开发强度较低的集水区和卫生基础设施。