Aquatic Weed Removal

Team Nemo is seeking a solution to the excessive weed growth in the University of Idaho Arboretum ponds. The ideal solution will likely include multiple components, so as to both answer the immediate problem and improve the long-term health of the pond. By addressing both the immediate and long-term, we hope to create an ecosystem-compatible pond maintenance process.

Problem Definition


Problem Statement

The excessive aquatic weed growth in the arboretum ponds is not aesthetically pleasing. We are looking for a long term, ecologically friendly, low-maintenance, and cost effective solution.

Background

Two small ponds were dug in the UI arboretum several decades ago. They were lined with bentonite and filled with natural runoff from rain and the arboretum irrigation system. The irrigation system runs reclaimed water, which is presumably nutrient rich. The ponds have low circulation volume (a pump at the bottom of the arboretum recirculates water back to the head of the upper pond through a 2" pipe.) These conditions encourage nutrient buildup and weed growth.

In the last few years the ponds have been infested with excessive growth of filamentous algae, Duckweed (Lemna minor), Water Meal (Wolfia sp.) and Mosquito Fern (Azolla sp.). The Azolla is a particular issue because it can fix nitrogen from the atmosphere, contributing even more to the nutrient buildup in the pond as it decomposes.

Previously Attempted Solutions

The Arboretum Horticulturist (Paul Warnick) has made several attempts to solve the weed problem, but none have achieved desired results.

 Project Design Goals 

1.	Reduce aquatic weed growth in Arboretum ponds.
 * -	Improve long term pond health
 * -	Have positive or neutral impact on surrounding ecosystem

2.	Be simple to use.
 * -	Low man hours
 * -	Safe and straightforward for possible volunteer use

3.	Meet budget provided by client

 Design Specifications 

Weed Types


Duckweed, water meal, Azolla, and filamentous algae are all common water weeds which prefer quiet nutrient rich water. All but filamentous algae are free floating plants, and in other situations can be valuable as fertilizer or animal feed.

Azolla is able to fix nitrogen into a usable form directly from the atmosphere through symbiotic relationship with cyanobacteria. Because it always has a ready supply of nitrogen, phosphorus tends to be the limiting nutrient. Azolla has a competitive advantage when Nitrate:Phosphate ratios are low.



Pond Chemistry
Ponds with low circulation tend towards eutrophication, or nutrient overload. This is especially true when they are subjected to run-off from fertilized lawns or fields, a definite condition in the Arboretum. Eutrophication encourages weed overgrowth, so removal or locking of nutrients in the system is a necessary part of pond maintenance. This can be done through removal of biomass (for instance weeding or harvesting fish), through draining and refilling, or through any biological or chemical process which causes nutrients to either escape or bond into an unusable form. Phosphorus is especially hard to remove because it has no gas phase.

Because Azolla has a competitive advantage when the nitrate:phosphate ratio is low, increasing the nitrogen levels may help discourage that particular infestation. If the ratio of dissolved inorganic nitrate to dissolved inorganic phosphate (DIN:DIP) is less than 10:1, Azolla likely has a competitive advantage. (Source: Dr. Wilhelm, UI College of Natural Resources) To find the DIN:DIP ratio, it is necessary to measure the concentration of nitrate and ammonia (components of DIN) and phosphate (DIP).

Pond Volume and Surrounding Topography
Pond volume is notoriously hard to estimate, but is necessary information for understanding the pond system and prescribing the "dosage" of many treatments. There are many sources online suggesting methods of calculating pond volume, since it is a common issue for owners wishing to fertilize their fish ponds. The recommendation from several fertilization guides and retailers was to simply estimate off surface area. For a more precise method, Penn State Extension recommended taking depth measurements on a grid, averaging them, and multiplying by the surface area of the pond. This was our method for finding initial pond volume estimates. Pond area was found from Google Maps (Daft Logic Google Maps Area Calculator Tool) and checked with a hand-held GPS by comparing reported perimeters.

The surrounding topography is also of interest, since it can provide information about the surface run-off area for the ponds. This could be used in calculations of expected refill or replacement time for pond water. Topographical data can be collected with a hand-held GPS. (Data can also be taken from Google Earth, but we were not confident on the vertical accuracy.)

Possibilities/Selection
Mechanical

- Remote Controlled "Water Plow" with Shoreline Conveyor Belt: Small boat with a mesh plow can be used to "herd" weeds towards the shoreline, where a conveyor belt facilitates scooping them out. (This is a scaled down version of a system we saw used by Weedo).

- Remote Controlled Collection: Small boat collects weeds and brings them to shore. This is also an idea which has been scaled down to fit a small pond. The biggest challenge with scaling this is maintaining sufficient buoyancy, since buoyancy (like volume) decreases with the cube of length. To address this a water-permeable mesh basket could be designed so that the captured weeds support their own weight.

- Manned Version of Plow or Collection: Either concept could be translated to a slightly larger manned version if our client would prefer simplicity. It did appear that this would make construction more expensive.

Long Term

- Buffer Zone: A swath of grass or other long-rooted plants around the pond could control nutrient and sediment inflow. This is a common and low maintenance water quality solution.

- Partial Pre-rain Drain: Partial draining of the pond system before expected rains could help renew the water. When rain falls on a full pond, much of the overflow will be the fresh rain. If the pond is lowered to receive the expected rainfall, old water is flushed and fresh water is retained.

- Nitrate Addition: Adding nitrates could reduce the Azolla problem, since Azolla has a competitive advantage in low nitrate to phosphate ratio environments. However, it would not address the long-term nutrient overload. Before proceeding, we would need confirmation that the arboretum ponds have a legitimately low nitrate to phosphate ratio.

- Aeration: Adding more aerators to the pond would increase the dissolved oxygen at the pond bottom. This would encourage the growth of aerobic bacteria which break down organic matter and improve water quality. It would also reduce pond odors, which would be valuable in the arboretum.

Chosen Designs
Mechanical

- Remote Control Convertible

Long Term

- Nitrate Addition

Document Archive

 * [[Media:16_NEMO_PreliminaryDesignReview.pdf|Preliminary Design Review (Fall 2016)]]


 * [[Media:16_NEMO_PhosphorusTest.jpeg|Preliminary Phosphate Testing]]


 * [[Media:16_NEMO_CombinedSchedules.jpeg|Combined Team Schedule (Fall 2016)]]