Keith Harries, principal hydrogeologist at Halcrow Water, looks at the relative effects of agriculture and wastewater disposal on nitrates in groundwater on the Caribbean island of Barbados.
Q1C1 + Q2C2 + Q3C3 = Q4C4 where Q = flowrate and C = nitrate concentration
A 1km wide test section was selected on the moderately developed south-east coast. The question posed was - can the observed rise in nitrates from inland to the coastline be attributed to wastewater disposal? The unknown in the equation is therefore C3 - the nitrate concentration in wastewater.
Once calculated, this figure can be compared to the expected concentration based on available data. Estimated or known parameters are:
- groundwater inflow and recharge to the coastal zone;
- wastewater discharge, based on population density in the coastal zone combined with a demand of 150l/capita/day;
- nitrate concentrations in groundwater as measured at inland and coastal sampling points.
The methodology was then applied to a more populous zone at South Point. This time, the value of 77mg/l was assigned for nitrate in wastewater and the equation was solved for the resultant nitrate concentration in groundwater at the coastline before discharge. The calculated concentration was 11mg/l. This compares with the average observed value of 10.7mg/l at a coastal monitoring borehole.
- the typical concentration of nitrate in groundwater beneath rural areas is 7-8mg/l, most of which (c.90%) is due to fertiliser application;
- moderate coastal development (<2000 population per km2) increases nitrates in groundwater discharging at the coastline by about 30%;
- only when population densities exceed 3000/km2, or where groundwater outflow is relatively low does the wastewater contribution to nitrate in groundwater match and exceed that from fertilisers; total nitrate concentrations can be up to 25mg/l.