Feature

Conveying Waste with Water

Natural hydrological processes offer a means of treating and transporting waste.

Earl Bardsley

A neglected branch of the hydrological cycle is the phenomenon of land-derived groundwater moving beneath the sea bed toward deeper ocean waters. This sub-ocean flow eventually emerges at submarine fresh water springs or seepage zones at varying depths on the ocean floor. Interestingly, the pressure of ocean water is not a controlling factor in limiting this groundwater discharge -- a submarine spring can flow equally well in shallow coastal waters or at the bottom of a deep ocean trench.

Total groundwater input to the world ocean amounts to some 2,000 cubic kilometres of fresh water every year, with some coastlines providing considerably more input than others. Three specific conditions are required for a significant offshore groundwater flow. First, there must be sufficient infiltration of rain or river water in the source region to maintain groundwater supply. Secondly, there must be permeable subsurface geological materials (aquifers) which allow ease of seaward water movement. Finally, the groundwater in the source area must have a sufficiently high elevation to provide a pressure gradient to push the water along in aquifers extending out beneath the sea. These conditions occur in many parts of New Zealand and some submarine springs are known, with no doubt many more to be discovered.

There has been little research to date into quantifying New Zealand's submarine fresh groundwater, despite its role in our water budget. Probably the lack of interest is because this water cannot provide a useful water supply. In principle, a line of pumped bores distributed along the coast could pull back the submarine groundwater for use on land, but this operation would not be sustainable because ocean water would then invade the aquifers and salt water would soon emerge from the bores.

Although offshore fresh groundwater offers little in the way of water supply, its seaward flow may nonetheless provide a valuable resource in a different context. In particular, the groundwater flow can be thought of as a convenient natural conveyer system for underground transport of treated effluent from land out to more distant ocean waters. The line of coastal bores is still needed, but this time they serve to pump effluent down rather than freshwater up.

Avoiding Pipeline Problems

In New Zealand and elsewhere a whole industry has developed around the traditional engineering approach of coastal effluent disposal via sea floor pipelines. Such outfalls are expensive to construct and repair, and cannot always be located far enough out to sea to avoid effluent being swept back to beaches from time to time. Also, the outfalls have to be constructed with a calculated risk of rupture during extreme storm events.

In contrast, all engineering works for groundwater disposal are located safely on shore, submarine pipeline costs are avoided, and the effluent may emerge much further out to sea than would ever be feasible with a pipeline. Also, when the groundwater effluent eventually does emerge, its quality is likely to have been improved by its long period of natural filtration beneath the sea floor.

The concept of submarine groundwater disposal could even be applied to effluent generated some distance inland. For example, one approach to remedying the "black drain" sector of the Tarawera River might be to construct deep bores for injection of the dark pulp and paper effluent from the Kawerau plant. Provided the bores were sufficiently deep, there is a good chance the effluent will not reappear until well out into the Bay of Plenty.

Of course, any given submarine groundwater disposal proposition would have to be carefully evaluated in the light of the local terrestrial and marine hydrogeology. Impervious basement rock would be quite unsuitable, nor would there be much gained by pumping effluent into an aquifer which discharged just beyond the surf zone. Also, there is additional cost involved in the pre-treatment of sewage or industrial effluent up to the quality required to avoid clogging of the injection bores due to build up of biomass or suspended particles.

Eliminating Effluent

Probably the most useful application of submarine groundwater disposal would be for removing treated sewage effluent from New Zealand coastal towns and cities in suitable hydrogeological situations. The list of localities where subsurface investigations might prove useful includes Tauranga, Gisborne, Napier, Hastings, and Christchurch.

Injection of effluent at these localities would also serve the dual purpose of increasing existing groundwater levels and reducing the risk of seawater intrusion from current groundwater use. Such treated-sewage barriers to seawater have been in place for some time along the California coast.

When treated effluent is pumped into a line of coastal bores there is also some landward movement of the effluent against the general direction of natural seaward transfer. This landward component allows the possibility of re-use of highly-treated sewage effluent water. Most New Zealanders would not wish to drink even the best quality water from the exit pipe of a sewage treatment plant, but the same water after passing through some distance of aquifer might be acceptable. A water recycling system of this type is currently in operation at the town of El Paso in Texas and a similar scheme might prove helpful for re-use of water in Auckland.

The use of some of our aquifers as effluent transportation resources may require some rethinking of the traditional protective attitude taken by regional councils towards aquifers. However, with the right planning and design, the sub-ocean floor transport system would appear to offer a useful opportunity to improve our physical environment.

Dr Earl Bardsley is senior lecturer in hydrology at Waikato University.