Ammonia Toxicity in a Leachate-Impacted Stream
A stream receives leachate from an unlined landfill. Upstream, total ammonia is 0.4 mg/L as N at pH 7.2. Downstream, total ammonia is 22 mg/L as N, pH 8.6, temperature 22 C, and dissolved oxygen 6.5 mg/L. A fish kill is limited to the first 300 m downstream. Chlorine and metals are below criteria. The laboratory reports an acute criterion of 0.05 mg/L un-ionized NH3 (as NH3) and gives pKa = 9.2 for ammonia at 22 C. The discharger argues the kill is from low oxygen because leachate has high biochemical oxygen demand, but the measured DO remains above 5 mg/L throughout the kill zone.
1. Calculate the un-ionized ammonia concentration downstream and determine whether it explains the fish kill. Show the fraction calculation and compare with upstream conditions.
Read the worked answer
The un-ionized fraction is:
f = 1 / (1 + 10^(pKa - pH))
Downstream: f = 1 / (1 + 10^(9.2 - 8.6)) = 1 / (1 + 3.98) = 0.201.
Un-ionized NH3-N = 22 mg/L x 0.201 = 4.42 mg/L as N. Convert to NH3: 4.42 x (17/14) = 5.37 mg/L as NH3.
Upstream: f = 1 / (1 + 10^(9.2 - 7.2)) = 1 / 101 = 0.0099. Un-ionized NH3-N = 0.4 x 0.0099 = 0.004 mg/L as N, or about 0.005 mg/L as NH3.
Downstream un-ionized ammonia is roughly 100 times the acute criterion and more than 1,000 times the upstream concentration. The measured DO of 6.5 mg/L does not support hypoxia as the primary cause. The spatial pattern, the pH shift, and the ammonia calculation together make ammonia the leading cause. Confirm with caged-fish or continuous monitoring, but the discharge is the obvious source.
2. The discharger proposes 5:1 dilution with cleaner upstream water to solve the problem. Evaluate whether dilution alone will meet the acute criterion.
Read the worked answer
Five-to-one dilution lowers total ammonia from 22 to about 4.4 mg/L as N, assuming complete mixing and no additional ammonia load. If pH stayed at 8.6, the un-ionized fraction would remain 0.201, giving 0.88 mg/L as N, or about 1.07 mg/L as NH3. That is still more than 20 times the 0.05 mg/L criterion. Dilution would need to be on the order of 100:1 or more at constant pH.
Mixing with upstream water at pH 7.2 may lower pH and reduce the un-ionized fraction, but the amount depends on alkalinity and buffer capacity, which are not provided. A simple pH average is not valid. Even if pH dropped to 8.0, the fraction would be about 0.059, and diluted total ammonia of 4.4 mg/L would still give about 0.26 mg/L as N, above the criterion.
The appropriate response is source control and treatment: reduce ammonia load, equalize and nitrify leachate, or adjust pH and strip ammonia. Dilution is not a treatment method and may violate anti-degradation requirements.