Most of the city's stormwater drains into the estuary including basically everything spilled on the ground - for example water runoff from roofs, dust off car brake pads, or anything spilled in a car accident or an industrial site.
Napier City Council's environmental solutions manager Cameron Burton said, "All those things that land off in a concreted area, basically, if they're not cleaned up at that time, will wash off into the estuary".
Looking out on the estuary on a typical sunny Hawke's Bay day, Ahuriri Estuary Protection Society secretary Sue Macdonald wondered if Napier residents were aware of this reality.
"As they say, if you drop litter in the CBD, and it goes through the system it will end up in the estuary and it's a big issue because it's sort of out of sight, out of mind with people, they don't have a comprehension," Macdonald said.
"There has been a little work done to try and mitigate this problem but like everything else we're on the back foot, it's got so bad that something has to be done."
On February 5, a hydrochloric acid spill of nearly 1000 litres caused chaos as it flowed from the industrial area of town by the estuary and into the waterways.
Council said hundreds of small fish and eels died.
There were three other significant spills that weekend - a paint discharge to the stormwater network, a wastewater overflow, and a fire in a scrap metal yard where all the fire water drained from the site, much of it reaching the estuary.
"It's not normal that there's such significant spills in a row like that, they take quite a toll on all the resources we have, " Burton said.
Those four spills are only a handful of 29 spills reported to the council so far this year - the most common are oil spills.
A council report suggested continued degradation of the estuary could result in "the system reaching an ecological point of no return".
Ngāti Kahungunu kaitātari matua, or senior environmental analyst, Shade Smith, said the estuary had traditionally been a food basket for tangata whenua.
"The principal cockle harvesting site was where the motorway bridge abutment was constructed at the southern end by the railway bridge, so that was bang straight on where the main cockle bed was," Smith said.
A Napier City Council urban waterways investigation found there was excessive phosphorus in water around Napier, up to 400 times the safe value for species protection.
Niwa states that phosphorus is an essential nutrient for plants and a natural component of healthy rivers, but too much can increase algae, causing deterioration.
Napier City Council environmental solutions manager Cameron Burton says spills "take quite a toll on all the resources we have".
Burton said it may or may not be an issue.
"If the species and creatures within the waterways have learned to assimilate that phosphorus, then it might not be a problem."
Smith said it could not be overlooked.
Algae was not so much of a problem when it was growing, but it could be a serious problem once it started decomposing, he said.
"Through the decomposition process, you get a reduction in the amount of oxygen within the sediments and you get potentially a dead zone in the estuary."
The council's urban waterways investigation will continue for two more years, hoping to gather more data to understand the reality of the city's waterways.
Burton said this would help the council make the right decision on how to tackle the problems.
"Rather than putting a big treatment wetland for instance off Prebensen Drive [in the suburb of Poraiti near the estuary], we're finding out whether it's the right thing to do, so we can do that for the right types of contaminants in the right kinds of places."
Burton said so far, no one had been charged after the acid spill - the council has six months to decide.
Spills reported to Napier City Council so far in 2021
What type?
Car crash - 6
Oil - 11
Diesel - 2
Paint - 1
Acid - 1
Process washwater - 1
Fertiliser - 1
Hazardous substance - 1
Concrete - 3
Fatty substance - 1
Petrol - 1
Where have they been in the city?
CBD - 2
Greenmeadows - 1
Marewa - 4
Mataruahou (Napier Hill) - 3
Meeanee - 1
Napier South - 1
Onekawa industrial - 1
Onekawa residential - 2
Pandora - 4
Parklands - 2
Pirimai - 3
Tamatea - 1
Taradale - 2
Westshore - 2
Total: 29 - Seven in January, 14 in February, and eight so far in March.
