6.2  Define what you are accounting for

Identifying relevant contaminants

As with water quantity accounting (see previous section), the first stage of water quality accounting is to define and establish what you are accounting for. The NPS-FM requires water quality accounting to address all relevant sources of contaminants. Careful consideration needs to be given when identifying which contaminants are relevant, as there could be major cost implications if irrelevant contaminants are selected. As a first step, it may help to understand whether the contaminants of concern are conservative or non-conservative. Non-conservative contaminants are those that are transformed in the environment. Understanding this will help to determine how the contaminants should be accounted for.

The NPS-FM requires councils to use the National Objectives Framework to set freshwater objectives and limits for an FMU. Because freshwater accounting is required for FMUs where freshwater objectives and limits are being set or reviewed, we briefly look at how some of the National Objectives Framework features help select relevant contaminants.

Compulsory values

Appendix 1 of the NPS-FM includes two compulsory national values – ecosystem health and human health for recreation – for which attributes are listed in its Appendix 2. The attributes relate to quantifiable aspects of the freshwater environment that councils must monitor and manage in order to meet the compulsory national values. Table 6.1 lists the attributes relating to these two compulsory values for both rivers and lakes. For example, to manage ecosystem health, the National Objectives Framework requires the management of total nitrogen (TN) and total phosphorus (TP), which would need accounting for in an FMU that contains lakes. Note that Appendix 2 of the NPS-FM is not fully populated and that additional attributes will continue to be added over time.

Table 6.1:      Attributes to be managed under the National Objectives Framework

Additional national values

As well as the compulsory values for each FMU, Appendix 1 of the NPS-FM contains additional national values that councils must also consider. There will be cases where an attribute listed in Appendix 2 for a compulsory value is also relevant to one of the additional national values. For example, E. coli could also be a relevant attribute for mahinga kai, mahi māra (cultivation), wai tapu (sacred waters) and wai Māori (water supply). It is also likely that other attributes, not currently listed in Appendix 2, may also be required to achieve these values. For example, an FMU that is deemed nationally significant with respect to mahinga kai might lie within a catchment used for coal mining. In this case, it may also be necessary to set freshwater objectives and limits for suspended sediment and heavy metals, and to include these contaminants in the freshwater accounts for that FMU.

Regional values

The NPS-FM also allows councils to set objectives and limits to protect other values that are not included among the additional national values in Appendix 1 (Policy CA2(b)(ii)). The setting of such freshwater objectives, and limits to achieve those objectives, will require accounting for the contaminants that impact on the values chosen.

One instance where the use of regional values (and unique attributes) will likely arise is in urban FMUs. For example, in the Auckland region, suspended sediment concentrations and heavy metals (copper and zinc) have been shown to be important, particularly in estuarine environments. While at first glance this may appear outside the remit of the NPS-FM, Policies A1, B1 and Appendix 1 specifically instruct councils to take into account the connections between water bodies, and connections between freshwater and coastal waters, in particular. Although the majority of the effect of these contaminants is on coastal ecosystems, the delivery mechanism is freshwater streams and creeks. This relationship is discussed further in case study 6.1.

The NPS-FM requires councils to set a freshwater objective for periphyton. Councils will manage periphyton biomass in streams and rivers through the use of limits on nitrogen and/or phosphorus from point and diffuse source discharges. To manage the periphyton attribute, dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) should be included in water quality accounts. The frequency of accounting for these contaminants may need to reflect seasonal variabilities in water temperature or flows (as discussed in 4.4). Similarly, although suspended solids and water clarity are not yet included as compulsory attributes for ecosystem health in the NPS-FM, there are numerous examples, particularly in lowland streams, rivers and estuaries, where they are significant factors impacting ecosystem health. Where regional freshwater objectives and limits are developed to manage sediment inputs to streams and rivers, then suspended sediment should be included in water quality accounts.

Another example of identifying relevant contaminants relates to fish species and their variable propensity to bioaccumulate contaminants. Bioaccumulation of contaminants may be an issue in some FMUs considered as nationally (or regionally) significant fisheries. The Rotorua lakes are one such example, where trout have been shown to bioaccumulate mercury, which potentially poses a health risk to people who consume more than a certain amount of trout. However, the source of the mercury is geothermal water, which the Bay of Plenty Regional Council can do nothing about. This is an example of ‘natural’ sources of contaminants, which are discussed further in 6.5. A council may choose not to set freshwater objectives and limits for mercury in such an FMU for this reason, but it is still important to account for this natural source of mercury. The inclusion of mercury (or other heavy metals) accounting may be justified in FMUs deemed significant fisheries where there are significant human-induced sources of the contaminant, where, although source control is not possible, other management actions such as health advisories could be used.

Tāngata whenua values

As with the compulsory values, additional values and regional values, measureable attributes that relate to any identified tāngata whenua values should also be included in freshwater quality accounts.

Units of freshwater accounting

The primary unit of water quality accounting is likely to be the load of contaminant (eg, kilograms or tonnes of nitrogen per year). This will be the means by which the amount of contaminant at a particular point in a stream network is compared with the amount attributable to different point and diffuse sources upstream of that point.

However, note that ecosystem response to contaminants is usually a function of the concentration of the contaminant in the water column. For example, the amount of phytoplankton in a lake is related to the concentration of nitrogen and phosphorous in the water column (as well as other factors, such as clarity). Concentration is the primary unit of any water quality analysis so there will likely be benefit in recording concentrations as well as load in a water quality accounting system. As flow will also be recorded for water quantity accounting purposes, instantaneous loads (concentration × flow) could be computed. However, whether concentrations are recorded inside or outside, the accounting system will depend on the method(s) used to compute loads and the temporal unit of accounting (ie, is it per day, week, month or year).

In most cases (at least initially), there may need to be paper-based calculations to compute point and diffuse source loads in those units. Where this is necessary, it may be more cost-effective to use existing databases to record concentration measurements (which are likely to be instantaneous measurements made at monthly intervals, for instance), computing loads and/or median concentrations, and then transfer the outputs to the water quality accounting ‘system’ (see 6.7). This can be done automatically within the accounting system, such as in case study 6.2.