Regional assessment of water quality trends in the Wellington region STATEMENT OF PRIMARY EVIDENCE OF ANTONIUS HUGH SNELDER ON BEHALF OF WELLINGTON REGIONAL COUNCIL February 2018

Basis of the evidence Analysis of water quality trends for rivers and lakes of the Wellington Region (Snelder, 2017b) Assessment of trends in 20 water quality measures at 62 sites in the region Analysis of regional-scale river water quality trends in the Wellington Region; period 2007 to 2016. (Snelder, 2017a) Helicopter view of regional water quality trends

Probability trend is improving Increasing DO “Virtually certain” 95% probability Decreasing DRP “Very likely” A trend analysis answers the question “what is the probability the trend is (say) improving”, given monitoring data. The plots are examples for a single site (Ruamahunga at Pukio). The probability DO is improving is 99%. We say its virtually certain to be improving. NOTE that increasing DO means improvement. The probability DRP is improving is 95%. We say its very likely to be improving. The probability NO3N is improving is 75%. We say its likely to be improving 75% probability Decreasing NO3-N “Likely”

Probability trend is improving or degrading Decreasing DRP “Unlikely” OR 78% probability increasing DRP “Likely”. At this site, the probability DRP is improving is 22%. We say its Unlikely to be improving NOTE: We could consider the probability that the trend is DEGRADING. In this case: we would consider it likely that DRP is degrading (because its probability is 78%)

Categorical levels of confidence Confidence water quality is improving 99–100 95–99 90–95 10–33 33–76 67–90 1–5 0–1 5–10 So to produce a visual summary of the probability of improvement across 62 sites and 20 water quality measures we can use the following colour codes. More green = more certain of improving trends. More red = reducing certainty of improving trends AND THEREFORE increasing certainty of degradation. Yellow = as likely as not (in other words the evidence both for and against improvement is evenly divided) So taking this colour scheme with us we go to a plot:

Probability that 10-year trends were improving 68% at least as likely as not improving The plot shows 62 sites (rows/horizontal) by variables (columns/vertical) For each Site + Variable the probability the trend is IMPROVING is shown by the colour. Recall more green = improving and yellow = as likely as not. So this plot is predominantly greens to yellow. This indicates overall 68% of these (10-year) trends are AT LEAST as likely as not improving. Note the result for 5-year was 66%. The sites + variable combinations that are orange to red are those which are more probably degrading. The results indicate that water quality has generally improved. They show that degradation has been relatively isolated. Some specific sites have degrading trends across several variables. 100 minus the proportion likely as not improving is the proportion that are at least LIKELY to be degrading.

Cumulative sites (%) with improving ten-year trends Variable No. sites Virtually certain Extremely likely Very likely Likely Likely as not Clar 52 46 62 75 88 90 Turb 56 18 27 34 55 66 DRP 35 11 23 26 49 TP 45 33 53 71 87 NO3-N 50 28 32 70 72 NNN 25 31 42 TN 40 60 68 82 92 TOC 51 6 16 22 E. coli 13 24 47 58 Fils-Max 4 17 Fils-Mean 76 Mats-Max 9 29 Mats-Mean 20 44 80 %EPT 2 73 %EPT_Taxa 54 41 57 MCI 7 63 QMCI This is Table 5 in evidence. Table 6 is the equivalent for 5 years. The proportion AT LEAST as likely as not improving by variable at shown in the last column Note all measures have more than 50% AT LEAST as likely as not improving. Note results for 5 years are Table 4 in my evidence. For 5 years three biological measures had LESS than 50% of sites AT LEAST as likely as not improving

Regional trends – Binomial tests Variable 10-year 5-year Clar Improving Turb Not Significant DRP TP NO3-N NNN TN TOC E. coli Not Significant* Chla Fils-Max Fils-Mean Mats-Max Mats-Mean %EPT %EPT_Taxa MCI QMCI A test of whether there is an “regional-trend” for the region was also performed using all trend results for each variable. The test considers if the number of sites that exhibited trends in a given direction were greater than could be expected if direction was random. A significant result is strong evidence of a systematic change in a given direction across the region. By “strong evidence” we mean high confidence that the observed change is not random (not a chance event), There are several significant IMPROVING regional trends in each time period There are NO significant DEGRADING regional trends in either time period Conclusion: there is a pattern of general regional improvement for some variables and NO pattern of regional degradation for any variables. The significance of the regional-trend was evaluated using a binomial test with p values adjusted for false discovery. The original report did not include this adjustment. This statistical issue was raised in the evidence of Adam Canning and I agreed. The correction however did not change the overall conclusion of this analysis. *Significant at the 10% level All p-values adjusted for false discovery

Conclusions Strong evidence of water quality improvement across the region over the past decade Water quality has not improved everywhere, But degradation is isolated rather than occurring in a consistent and regional scale manner. Expert conferencing agreement that: The exceptions to improving trends should be noted P7.5 in evidence It is important to distinguish trends from current state Trends can be improving but from a degraded current state A particular exception is for 5 year trends, three biological measures had LESS than 50% of sites AT LEAST as likely as not improving Periphyton and invertebrates are sensitive to water quality and are used as long term integrative measures of water quality and other impacts. Given that physical and chemical measures of water quality are generally improving, the reasons for declines in these measures are unclear. It is noted that the were not degrading regional trends for these, or any other variables.