Professor Isaac R Santos

Nitrogen leaching from agricultural land use is known to impact Coffs Harbour stream water

quality. On-farm denitrifying woodchip bioreactors can potentially mitigate nitrogen,

particularly nitrate (NO3-) pollution by maximising denitrification capacity in between farms

and creeks. However, denitrification may release the powerful greenhouse gas nitrous oxide

(N2O), swapping from aquatic (NO3-) to atmospheric (N2O) pollution.

Here, we assess nitrate nitrogen (NO3--N) removal and N2O emissions from a new edge-of field

surface-flow bioreactor design during ten rain events on intensive farming land.

Our nitrate removal rates (NRR) varied between 5.4 and 76.2 g NO3--N m-3 wetted woodchip

d-1 with a mean of 30.3±7.3 g NO3--N m-3 d-1. The nitrate removal efficiency (NRE) ranged

from ~73% in ideal conditions to ~18% in non-ideal conditions. Overall, 9.9 kg NO3

--N ha-1 yr-1 were removed via the bioreactor, representing an overall 30% efficiency when

incorporating all flow and overflow events. However, inflows and treated outflows from the

bioreactor are ~254 and ~138 fold higher than ANZECC guideline values, respectively,

indicating that there is still a significant risk to local waterways from treated effluent.

The bioreactor did not contribute higher N2O emissions than what naturally occurs if the

bioreactor was absent, implying minor swapping from aquatic to atmospheric pollution. NO3

--N that was removed in the bioreactor and converted to N2O (rN2O) was ~3.3 fold lower than

the expected 0.75% IPCC emission factor.

Our modelled NO3--N removal from the bioreactor would cost AUS$17.8 per kg NO3

--N removed. Whilst off-farm NO3- losses are expected, even under best management practice,

the removal cost using this bioreactor is ~5 fold greater than the estimated cost of nitrogen

fertiliser application. Reducing on-farm NO3- use to lower environmental losses may be more

cost-effective than treating effluents; however, the combination of both management

techniques is likely necessary to protect environmental assets.

With minor design modifications, the bioreactor's efficiency will likely increase. Overall,

edge-of-field surface-flow bioreactors can be a useful tool to reduce NO3--N runoff in eastern

Australian intensive horticulture catchments and play an integral role in the suite of NO3

--N management solutions. This bioreactor represents a proof-of-concept and a new tool to

protect vital aquatic habitats such as the Solitary Islands Marine Park.

Recent studies on the Coffs Coast linked water quality issues to agricultural intensification.

Runoff from hothouses can contain high levels of nitrogen (N) from fertilisers often draining into

streams without any treatment. To manage nutrient pollution from hothouse drainage, we tested whether

a new woodchip bioreactor approach can attenuate the highly concentrated N effluent.

The experimental trial was designed and deployed as a collaboration between Southern Cross

University, North Coast Local Land Services and Coffs Harbour City Council. Here, we investigate the

effectiveness of buried, inline woodchip bioreactors in removing nitrate (NO3

--N) and the possibility of

pollution swapping by producing the greenhouse gas nitrous oxide (N2O).

Flow rates through the bioreactors across the 5 surveys were 65±29 L hr-1, and residence time

was 6.4±2.6 hrs, whilst water temperature was 17±1.0°C. Average dissolved NO3

--N removal and N2O

gas production were 6.0±5.2 g N m-3 hr-1 (0.9-12.3 g N m-3 hr-1), and 35.1±31.4 mg N m-3 hr-1 (14-83 mg

N m-3 hr-1), respectively. Average water depth was 8 cm, and only 21% of the bioreactor woodchips were

wet. A significant correlation between dissolved oxygen with NO3

--N and N2O-N indicated higher

removal occurred in hypoxic to anoxic conditions.

Overall, these inline pipe bioreactors achieved nitrate removal efficiencies (NRE) of 14.5±6.8%

(8.2%-25.0%) and N2O production equivalent to 0.7±0.6% (0.3-1.4 %) of nitrate removal. Our results

imply that these trial bioreactors are not significantly swapping NO3

--N removal with increased N2O

production to the atmosphere. Design improvements are required for better NO3

--N removal and a

review of on-farm fertigation is needed to minimize losses downstream. A suite of measures will be

required to achieve treatment levels that meet water quality guidelines.

We performed dissolved trace metal investigations in Double Crossing Creek, a tributary of Hearnes Lake estuary and part of the Solitary Islands Marine Park (SIMP), to assess the potential influence of horticultural activity and rain events on water quality.

Time series measurements of dissolved trace metals was undertaken from 27 January to 3 April 2018 over multiple hydrological regimes. After a 109 mm rain event on 25 February, Hearnes Lake began to drain to the ocean. After the estuary drained, streamflow past our sample site was exclusively from the upstream catchment.

The concentrations and export rates of dissolved trace metals in Double Crossing Creek increase after rainfall events of various magnitudes. Rain events of 30 mm were sufficient to increase dissolved contaminant concentrations above the Australia and New Zealand Environment and Conservation Council (ANZECC) water quality guideline (WQG) values for both fresh and marine water.

Generally, export rates of dissolved contaminants were greatest when rainfall exceeded 50 mm. The elements mercury (Hg), copper (Cu), and zinc (Zn) exceeded either the ANZECC freshwater or marine WQG for more than 20 % of sampling events, in both wet and dry conditions. During and within 24 hours after rain events, Hg and Zn exceeded ANZECC WQG by more than 10 fold during our sampling. Flushing of agricultural soils containing these trace metals is believed to be the source of dissolved Hg, Cu, and Zn to Double Crossing Creek.

Estimated export rates of dissolved trace metals were high compared to examples from the literature on a per area basis.

Comparisons of trace metal export from Double Crossing Creek to subsequent sediment burial rates of trace metals in Hearnes Lake revealed the estuary may be a source of dissolved Hg, Cd, and Mn to a sanctuary zone of the SIMP at times when the estuary is hydrologically connected to the ocean. At times when Hearnes Lake is closed, the estuary is believed to retain upstream inputs.

No data on the ecological effect of trace metal export was collected. We suggest analyses of the chemical speciation and biological accumulation of trace metals (especially Hg) is undertaken to better understand the ecological implications of trace metal export from this horticultural catchment to the SIMP.

To prevent dissolved contaminant exposure in areas downstream of intensive horticulture we recommend actions to reduce use of products which may contain Hg, Zn, and Cu and to minimize and capture runoff during rain events.

Southern Cross University performed soil and sediment quality investigations on farms with

various land use histories, including blueberry and banana cultivation, with funding from the

Coffs Harbour City Council’s Environmental Levy program.

Terrestrial soils from chemical mixing sheds, crop production areas (fields), drainage areas,

and bottom sediments from retention reservoirs (dams) were sampled from three different

sites in the Coffs Harbour local government area. Sites 1 and 2 were in the Sandy Beach

catchment, while Site 3 was in the Bucca Bucca Creek catchment. All three sites are currently

used for blueberry cultivation.

Terrestrial soil cores were analysed for 97 different pesticide residues of different classes

(herbicides, insecticides, fungicides). Additionally, terrestrial soil and dam sediment cores

were analysed for trace metal contents.

Spatial distributions of pesticides and trace metals in terrestrial soils and sediments were

analysed to identify contaminated sites. Comparisons to the Australia and New Zealand

Environment and Conservation Council (ANZECC) and other soil quality guidelines (SQG)

and enrichment factors (anthropogenic vs. naturally occurring portions) were used to assess

contamination of trace metals.

The farm in the Bucca Bucca catchment (Site 3) had the greatest degree of contamination

with pesticides. Fifty percent of the detectable pesticide residues were from two cores at this

site. Ninety-eight percent of the total detected pesticide mass was observed in the chemical

mixing shed core from Site 3. The fungicides propiconazole and cyprodinil were the most

abundant pesticides from this core. Soil contents of these fungicides should decline as they

degrade via natural processes, however propiconazole and cyprodinil can persist in the

environment while bound to soil organic material. Degradation rates remain uncertain.

Additionally, toxicity of propiconazole and cyprodinil degradation products may be greater

than their parent compounds.

14 of 26 subsamples of terrestrial soil and dam sediment cores from Site 2 in Sandy Beach

had arsenic contents over the SQG low value (20 mg kg-1). One of these samples exceeded

the SQG high range value (70 mg kg-1). Arsenic is suspected to be from pesticide application

which occurred during previous land use (banana cultivation).

Subsamples from dam sediment cores at all three sites had mercury (Hg) contents over the

SQG low value (150 μg kg-1). Mean contents of Hg from dam sediments (141 ± 15.5 μg kg-1)

were greater than terrestrial soils (78 ± 6.5 μg kg-1) suggesting mobility of Hg from terrestrial

soils and retention in nearby sediments.

Overall, based on their environmental toxicity, distribution, persistence, and mobility we

identified the herbicide diuron, trace metals cadmium (Cd) and (Hg), and the fertiliser

nutrient phosphorus (P) as contaminants of high concern. Using international pesticide

toxicity standards from the Globally Harmonised System of classification and labelling of

chemicals, we classified all other detected pesticides as contaminants of potential concern.

We highlight that our sampling was spatially restricted and suggest more sampling of farm

soils to increase confidence in identifying contaminants of concern.