Journal of Hydrology (NZ) 50 (2): 313-338 2011 © New Zealand Hydrological Society (2011) Nitrate sources and residence times of groundwater in the Waimea Plains, Nelson Michael K. Stewart1, Glenn Stevens2, Joseph T. Thomas2, Rob van der Raaij3, Vanessa Trompetter3 1 Aquifer Dynamics & GNS Science, PO Box 30368, Lower Hutt, New Zealand. Corresponding author: [email protected] 2 Tasman District Council, Private Bag 4, Richmond, New Zealand 3 GNS Science, PO Box 30368, Lower Hutt, New Zealand Abstract the various wells. The timing of the derived Nitrate concentrations exceeding Ministry of nitrate input history shows that both the Health potable limits (11.3 mg/L nitrate-N) diffuse sources and the point source were have been a problem for Waimea Plains present from the 1940s, which is anecdotally groundwater for a number of years. This work the time from which there were increased uses nitrogen isotopes to identify the input nitrate sources on the plains. The large sources of the nitrate. The results in relation piggery was closed in the mid-1980s. to nitrate contours have revealed two kinds Unfortunately, major sources of nitrate of nitrate contamination in Waimea Plains (including the piggery) were located on groundwater – diffuse contamination in the the main groundwater recharge zone of the eastern plains area (in the vicinity and south plains in the past, leading to contamination of Hope) attributed to the combined effects of the Upper and Lower Confined Aquifers. of the use of inorganic fertilisers and manures The contamination travelled gradually for market gardening and other land uses, northwards, affecting wells on the scale of and point source contamination attributed to decades. Input of nitrate to the groundwater a large piggery to the south of Hope. has been decreasing since about 1988 due Once nitrate is introduced to a groundwater to closure of the piggery. The resulting system it can take many years for it to be decrease in nitrate concentrations is flushed out. Tritium measurements in wells now also gradually travelling northward. are interpreted to give mean residence times, Groundwater to the south and west already and the spread of residence times around the had relatively low nitrate because of river mean, for groundwater in different parts of and/or rainfall recharge with low nitrate the plains. Mean ages are youngest in the area concentrations. Improved monitoring and south of Hope, where nitrate concentrations practices (e.g., best management practices are highest, and increase to the south, west and nutrient budgeting) need to be and north. The age distributions have been encouraged among market gardeners and used to produce a nitrate input history for other land users, taking special account of the Upper and Lower Confined Aquifers the location of the groundwater recharge by simulating the nitrate measurements in areas around and south of Hope. 313 Keywords Identification of nitrate sources by means Age dating; nitrate concentration; nitrogen of nitrogen isotopes has been applied by a isotopes; tritium; groundwater flow number of researchers (e.g., Fogg et al., 1998; Kendall, 1998). δ15N values often allow good discrimination between animal sources and Introduction inorganic fertiliser or natural nitrate sources, The Waimea Plains (lying southwest of but not between fertiliser and natural sources Nelson City) are an area of intensive farming or between animal and human sources. and horticulture of some 75 km2 (Fig. 1). Variability of δ15N values within a single Groundwater from unconfined and two source type limits the ability to distinguish major confined aquifers is used extensively between sources, and groundwater nitrate can for irrigation. The Hope Gravel Lower often come from more than one source type, Confined Aquifer unit supplies a large part both adding to the possible confusion. of the township of Richmond’s drinking Tritium and CFC concentrations have been water supply (Johnston, 1979; Dicker, 1980; used to determine groundwater residence Fenemor, 1988). Summer drawdown has times, which show when the water and a significant effect on usage and optimum dissolved nitrate entered the system (Stewart allocation of the resource is an issue. Water and Morgenstern, 2001). Tritium (T=3H) quality is generally good, except for nitrate is a component of the water molecule and concentrations which often exceed the limit therefore almost an ideal tracer. It is produced for potable waters (11.3 mg nitrogen/L H2O, naturally in the atmosphere by cosmic rays, Ministry of Health, 2005). This paper uses and large amounts were released into the geochemical methods to investigate the atmosphere by nuclear weapons tests in sources of nitrate and its residence times in the late 1950s and early 1960s, giving rain groundwater in the plains. and surface water a relatively high tritium Nitrate concentrations higher than the concentration, particularly in the Northern drinking water standard are not uncommon Hemisphere. In the Southern Hemisphere, in groundwater in New Zealand, and many there is now almost no bomb-peak tritium other parts of the world. Several surveys of remaining in groundwater systems, either nitrate concentrations in groundwater in because of radioactive decay and dispersion, or because bomb-tritium has completely the Waimea Plains have been carried out passed through short-residence-time (Stanton and Martin, 1975; Fenemor, 1987; hydrological systems. Instead, groundwater Dicker et al., 1992; Ware, 2000; Stevens, can now be dated using the natural decay 2005). With intensification of farming and of cosmic ray tritium because sufficiently horticulture, better understanding of nitrate precise tritium measurements are available sources is needed to allow better management (Morgenstern and Taylor, 2009; Stewart of groundwater resources. There are many et al., 2010; Morgenstern et al., 2010). possible sources of nitrate contamination Other tracers (CFCs and SF6) are providing within groundwater systems, such as natural new methods of age-dating groundwater soil organic matter, inorganic nitrogenous (Plummer and Busenberg, 1999). The steady fertilisers, dairy and pastoral farming, animal increases of CFCs and SF6 in the atmosphere manure applied to land (dairy shed effluent, since 1940 and 1970 respectively potentially chicken manure, blood and bone), septic tag subsurface waters with their dates of last tanks and municipal sewage effluent. Which exposure to the atmosphere. of these are the major nitrate contributors is Being able to combine identification of often not clear. nitrate sources with determination of when 314 Figure 1 – Map of the Waimea Plains showing the locations of wells sampled for nitrogen isotopes, and tritium and/or CFC concentrations. The trace of the cross section in Figure 2 is shown as a dashed line. the nitrate entered the groundwater system between groundwater nitrate and land use, offers a powerful means of investigating and contribute to better understanding and nitrate contamination problems (Böhlke management of the groundwater. and Denver, 1995, Katz et al., 2001; Moore et al., 2006). The objectives of this work are Previous work to investigate the input sources of nitrate to Hydrogeology Waimea Plains groundwater using nitrogen The late Quaternary terrestrial gravel isotopes, to estimate groundwater residence underlying the Waimea Plains, which is times using tritium concentrations, and to up to 60 m thick, has been divided into combine them to deduce the history of nitrate several formations (Johnston, 1979). These input on the plains and infer future changes. unconformably overlie clay-bound Moutere This will help to establish the connections Gravel of late Pliocene or early Pleistocene 315 age. Neither the Moutere Gravel nor older Gravel to form a sea cliff up to 12 m high that formations are considered likely to yield extends eastward from the Waimea River near appreciable quantities of groundwater. Only Appleby towards Richmond. On the seaward the Hope Gravel (late Pleistocene) and side of this cliff, while the Appleby Gravel was Appleby Gravel (Holocene) are important as being deposited, small streams draining the sources of groundwater (Fig. 2). eastern part of the Waimea Plains deposited The Hope Gravel occupies most of the relatively poorly-sorted gravel mapped as the valley cut in the Moutere Gravel by the Pugh Gravel Member of the Appleby Gravel. Waimea River and is mainly a tight, poorly The Appleby Gravel is generally highly sorted, clay-bound gravel up to 55 m thick permeable and forms unconfined aquifers (Fig. 2). However at two levels beneath the (referred to as the Appleby Gravel unconfined Plains it contains large lenses of sorted, less aquifers or AGUA). The gravel is up to 14 m clay-bound gravel which, along with minor thick on the coast but thins inland to 6 m thick lenses elsewhere in the formation, contain in the lower Wai-iti valley and 3 m thick at confined aquifers. In the east, Stoke Fan the Brightwater Bridge over the Wairoa River. Gravel derived from the Barnicoat Range The unconfined aquifers rest unconformably (500 m) to the east of the plains interdigitates on the clay-bound gravel with no appreciable with and partly overlies the Hope Gravel loss of water into the Hope Gravel, except (Fig. 2). near the mouth of the Wairoa Gorge and near After deposition of the Hope Gravel, the Appleby. The Pugh Gravel Member of the rivers cut down into and reworked the surface AGUA lies adjacent to the coast between the gravel and deposited the well-sorted Appleby Waimea River delta and Richmond. Water Gravel underlying the floodplains of the Wai- yield from the aquifers is generally high, but iti, Wairoa and Waimea Rivers and in the delta in the Pugh Gravel Member permeability and of the Waimea River. Adjacent to the coast consequently water yield decreases rapidly in the sea removed the upper part of the Hope an easterly direction. Figure 2 – Schematic cross-section from NW to SE through the central part of the Waimea Plains showing late Quaternary units (from Dicker et al., 1992).