Effective depth controls the nitrate removal rates in a water supply reservoir with a high nitrate load

The Occoquan Reservoir is part of an indirect potable reuse system where a water reclamation plant (WRP) discharges a nitrified product water to prevent the onset of anaerobic conditions in the bottom sediments during the summer months. The elongated narrow shape of the reservoir combined with water temperature gradients in the inlet results in density currents that enhance the transport of nitrate from the surface to the bottom waters. The morphology of the reservoir also causes a longitudinal change in the ratio of water volume to sediment area, herein defined as the effective depth (Z_ED). Field observations revealed that first-order nitrate removal rate coefficients (k) varied inversely with Z_ED, suggesting that the upper reaches of the reservoir have a higher potential for nitrate removal compared to the areas closer to the dam. A similar relationship between k (d^-1) and Z_ED was confirmed during laboratory experiments. Differences in k values were attributed mainly to the change in the nitrate supply rate as a result of the increase in water volume flowing over a specific sediment area, which limited nitrate transport to the sediments. The low variability found between the mass transfer coefficients for nitrate (Coefficient of Variation = 0.25) suggested a nearly constant biotic nitrogen removal and confirmed that k values were mainly affected by changes in Z_ED. Finally, similarities in k values between field and laboratory samples with similar Z_ED values suggested that different segments of natural systems may be properly downscaled to laboratory-sized configurations for analytical purposes by means of the Z_ED concept.