Risks from Nitrate & Pesticides to Groundwater. Murray Close

Transcription

1 Risks from Nitrate & Pesticides to Groundwater Murray Close

2 Why is nitrate a contaminant of concern? Drinking water health hazard Maximum Acceptable Value 11.3 mg N-NO 3 /L Environmental hazard Eutrophication of surface waters, incl. toxic algal blooms Eco-toxicity as low as 0.7 mg N- NO 3 /L

3 Why is nitrate an elusive pollutant? NO 3

4 Sources of Nitrate in NZ Septic tanks Point source Waste effluent disposal fields, e.g.: Sewage works Abbatoirs Dairy factories Diffuse source Agriculture: Grazing animals, esp. from urine Tillage Fertiliser application for pasture & crops Leaking effluent storage ponds

5 Relative impacts of different land-uses Land-use Typical nitrate conc. [mg N/L] Mass leached nitrate [kg N/ha/yr] Native forest Exotic forest Viticulture Apple Berry fruit Dryland sheep Dryland deer Dairy (3 cows/ha; winter off) Lifestyle block Dairy (3 cows/ha; winter on) Crops (mixed) Adapted from Table A2 in: Lilburne et al. (2010) Environment Canterbury Report R10/27 Values reflect estimates for extra light soil for a Lincoln climate

6 Morgenstern, U. and Daughney, C. (2012) Groundwater age for identification of baseline groundwater quality and impacts of land-use intensification 6

7 MFE SOE results: Median nitrate ( ) 7

8 Spatial (depth) dependency B B e.g. Waimakariri Lake Ellesmere: recent (Holocene) fan deposits Waimakariri R. underflow (no NO 3 ) Waimakariri R. Nitrate low redox zone; low nitrate B B Lincoln Lake Ellesmere HIGH River recharge Land surface recharge LOW Relative concentration Hanson and Abraham (2009) ECAN REPORT R09/39 8

9 Heterotrophic denitrification 1) Natural process carried out by heterotrophic facultative bacteria 2) Organisms breathe nitrate in lieu of oxygen 3) Require food (energy) source e.g. organic carbon 4) Harmful nitrate converted to innocuous N 2 gas N 2 gas bugs carbon food nitrate

10 Permeable reactive barriers (denitrification walls) - target shallow groundwater Entrench solid organic carbon Photo: Louis Schipper source below water table K PRB >> K aquifer Robertson et al. (2005) Groundwater v43 (4):

11 Pesticides in Groundwater New Zealand agricultural context (2004 data) Land use Area% Total tonnes ai per yr Mean loading (kg ai/ha/yr) % Total use Pastoral 79% % Forestry 18% % Arable 1.5% % Horticulture 1.2% %

12 Pesticides in Groundwater Pesticides are used extensively in primary production and can move off-site impact surface waters via run-off Impact groundwater via leaching Unlike nitrate, pesticides have a wide range of leaching behaviour Key parameters: mobility (Koc) and persistence (half-life) Different pesticides will impact groundwaters compared to surface waters

13 Pesticide leaching studies: approach Selected 7 sites to give range of soil types and climatic conditions Applied a suite of pesticides together with tracers Monitored their movement using suction cups and soil sampling for between 15 and 39 months Used inverse modelling to obtain leaching parameters

14

15 Hawkes Bay: Ngatawara Rd

16 Hawkes Bay: Watson Rd Hamilton site

17 Bromide: observed & simulated conc. in soils & GW at Hamilton site

18 Bromacil simulations - Southland

19 Summary leaching studies Most pesticides had similar mobility & persistence Four pesticides (diazinon, picloram, procymidone, & triclopyr) had greater mobility &/or persistence than indicated from literature hence a higher probability of leaching Allophane - variable charge - also adsorb -ve ions: weak bases (eg. triazines) less adsorbed; weak acids (eg. picloram) more adsorbed significant effect for hexazinone, picloram & triclopyr Close et al.(2008). Field studies of pesticide leaching. NZ J. of Crop and Horticultural Science, Vol. 36: 9 30

20 National surveys Surveys in 1990, 1994, 1998, 2002, 2006, 2010 & to assess if there was a problem; limited to 6 areas 1994 : more widespread 1998 & 2002 : every region : Most regions Surveys analysed for acid herbicides & semi-volatile organics - range of organochlorine, organophosphorus & organonitrogen pesticides 1990 & 1994 : DL around 0.1 ppb 1998 to 2014 : DL around 0.01 ppb

21 Pesticides in NZ Groundwater Year # of wells DL = 0.1 ppb 7% 14% 11% 9% 8% 7% 10% DL = 0.01 ppb 35% 21% 19% 24% 17%

22 Detailed results from 2014 survey 13 regions took part in 2014 survey 28 wells with pesticides detected (17%) 10 wells had 2 or more pesticides detected 21 different pesticides were detected 86% of detections were herbicides 61% of detections were triazines Most detections were < 0.1 ppb; 4 were > 1 ppb

23 2014 survey continued One well with dieldrin > MAV (0.043 cf 0.04 ppb) Next highest was terbuthylazine at 17% MAV; rest of pesticides were 5% or less of MAV No pesticides have been detected in Hawkes Bay region - 6 surveys from 1994 onwards Overall fairly constant frequency of low level detections Most detections well below MAVs Next survey is due at end of 2018

24 Murray Close T: E: 2014 ESR