Nitrate pollution of surface water from farms and urban runoff is widespread – impairing drinking water supplies, recreation, and wildlife habitat. The scale of the problem in rivers has overwhelmed most watershed-based best management practices. A new paradigm is needed. In this paper we highlight the 119 ha (294 acres) Prado Wetlands in southern California, USA as an example of a gravity-enabled, free surface flow vegetated wetlands to treat a river – the nitrate-rich, effluent-dominated Santa Anna River. The wetland treats ∼1–2 m³/s of the 7.1 m³/s (250 cfs) flow of the river in 28 wetland cells in two parallel trains (residence time 2–4 d, hydraulic loading rate 10–20 cm/d). The cleaner water is returned to the river and infiltrated downstream into a managed groundwater basin supplying 2.4 million customers. Since 1998, ∼1200 t of nitrate-N have been permanently removed from the river and released as N₂-gas into the air. Over 24 years, in the warm summers an average of 6.1 mg-N/L entered the wetland and 3.7 mg-N/L flowed out, a removal efficiency of 57%. In the cooler winters, an average of 6.4 mg NO₃-N/L entered and 2.6 mg NO₃-N/L flowed out, a removal efficiency of 45%. Individual months varied widely in concentration removal efficiency from 7% (winter) to 98% (high summer). Areal summer removal averaged 137 mg NO₃-N/m²/d and the apparent “k” was around 46 m/y. All-year rates were 129 mg NO₃-N/m²/d or 47 g/m²/y, a reasonable performance considering the size of the system and its configuration for both nitrate removal and duck hunting (i.e., more open water). Annual average nitrate removal efficiency increased substantially over time (45 to 72% for 1998–2004 versus 2016–22) and is attributed to an increase in hydraulic residence time, upgraded internal plumbing, and increased winter temperatures (+3C°). Prado Wetlands also removes phosphorus, though not as well as nitrate; average warm season removal efficiency was 23% (inflow 1.20 mg PO₄-P/L, outflow 0.93 mg PO₄-P/L). The aerial removal rate was high at 23 mg P/m²/d or 8.5 g/y, probably due to the high inflowing P-concentrations and the hard water. Wildlife was also enhanced. Least Bell's Vireo (Vireo belli pusillus), an endangered bird in the Prado Basin, increased from 19 pairs (1998) to a recent average of 444 pairs due to habitat improvement and reduction of cowbird parasitism. Results from the Prado Wetlands show that cleaning up rivers via diversion through wetlands benefits both water quality and wildlife.

农场和城市径流对地表水的硝酸盐污染十分普遍，损害了饮用水供应、娱乐和野生动物栖息地。河流问题的规模已经压倒了大多数基于流域的最佳管理实践。需要一个新的范式。在本文中，我们重点介绍美国加利福尼亚州南部 119 公顷（294 英亩）的普拉多湿地，作为利用重力、自由表面流植被湿地处理河流（富含硝酸盐、以污水为主的圣安娜河）的示例。湿地在两个平行列车的 28 个湿地单元中处理7.1 m ³ /s (250 cfs) 河流流量中的 ~1–2 m 3 /s（停留时间 2–4 d，水力负荷率为 10–20 cm ^/ d). 清洁的水返回河流，并渗入下游管理的地下水盆地，为 240 万客户提供服务。自1998年以来，约1200吨硝酸盐-N已从河流中永久去除并以N ₂气体形式释放到空气中。24年来，在温暖的夏季，平均有6.1 mg-N/L进入湿地，3.7 mg-N/L流出，去除效率为57%。在较冷的冬季，平均进入6.4 mg NO ₃ -N/L，流出2.6 mg NO ₃ -N/L，去除效率为45%。各个月份的浓度去除效率差异很大，从 7%（冬季）到 98%（夏季最高）。夏季面积平均去除量为 137 mg NO ₃ -N/m ² /d，表观“k”约为 46 m/y。全年排放率为 129 mg NO ₃ -N/m ² /d 或 47 g/m ² /y，考虑到系统的规模及其用于硝酸盐去除和猎鸭的配置（即更多的开放水域），这是一个合理的性能）。年平均硝酸盐去除效率随着时间的推移大幅增加（1998-2004 年与 2016-22 年相比，增加了 45% 至 72%），这归因于水力停留时间的增加、内部管道的升级以及冬季温度的升高 (+3C°)。普拉多湿地也能去除磷，但硝酸盐的去除效果不那么好。暖季平均去除效率为23%（流入量1.20 mg PO ₄ -P/L，流出量0.93 mg PO ₄ -P/L）。空气去除率高达 23 mg P/m ² /d 或 8.5 g/y，可能是由于流入的 P 浓度高和硬水所致。野生动物也得到了加强。由于栖息地的改善和牛鹂寄生的减少，普拉多盆地的濒临灭绝的鸟类小贝尔绿雀（Vireo belli pusillus ）从19对（1998 年）增加到最近的平均 444 对。普拉多湿地的结果表明，通过湿地改道来清理河流对水质和野生动物都有好处。