Tracking nitrates

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.

Historically, the main industry on Barbados has been agriculture, notably sugar cane, but in recent years this has been overtaken as the principal money-earner by tourism. Tourism is largely centred on the south-west coast, and along the west coast to the north of Bridgetown. In contrast, the east coast is less developed and exposed to rolling Atlantic waves.

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 value for C3 calculated from the equation was 77mg/l. In Barbados, the nitrogen loading per capita is 5kg/year. Assuming that this is all oxidised to nitrate in the groundwater environment, then such a loading diluted by the per capita wastewater discharge gives a nitrate concentration of 90mg/l in the effluent. Thus, the two values are of the same order.
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.

Conclusions

  • 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.


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