First RCE Thematic Conference: Towards Achieving the SDGs 5-7 December 2017 Okayama, Japan Field Trip to Okayama City

1. Beautiful Lake Kojima

2. Water quality of Lake Kojima
The water quality (chemical oxygen demand: COD) of Lake Kojima has been slowly
improving over decades, albeit with fluctuations. However, it has not been reduced
to the environmental standard (5 mg/L) yet.
Also, neither total nitrogen (T-N) nor total phosphorus (T-P), which are two
indicators of eutrophication, has been reduced to the environmental standard
(1.0 mg/L and 0.1 mg/L, respectively).
Secular changes in COD concentration
Secular changes in total nitrogen concentration (annual mean)
Secular changes in total phosphorus (annual mean)
Concentration
Water quality target (75th percentile)
set in the seventh-term plan
FY
75th percentile
Annual mean
Water quality target (75th percentile)set in the seventh-term plan
Environmental standard
Environmental standard
Concentration
FY
Water quality target (75th percentile)set in the seventh-term plan
Environmental standard
Concentration
FY

3. Measures to conserve the environment of Lake Kojima
(1) Lake Kojima water quality conservation plan
The seventh-term plan for lake water quality conservation
(overview)
■ Plan period
From FY 2016 to 2020
■ Targets to achieve
Item
FY 2020
(target)
FY 2016
(result)
total nitrogen concentration (annual mean)
total phosphorus (annual mean)
COD(total nitrogen)

4. (2) Recycling of agricultural water
Since 1978, Okayama Prefecture, Okayama City, Kurashiki City, and Soja City have
been conveying agricultural water to Lake Kojima to conserve the lake’s water
quality. A survey by Okayama Prefecture estimates that water conveyance of
100,000 m3 per day for one year will improve the lake’s COD by 0.4 mg/L.

5. A reed community absorbs nitrogen and phosphorus from lake water to help improve the water
quality. It also plays an important role as a breeding place for birds and fish.
Okayama Prefecture has been carrying out a Lake Kojima reed reaping and recycling project to
conserve the reed communities there.
(3) Lake Kojima reed reaping and recycling project

6. (4) A project for promoting a shift to low P-K fertilizers
Some thirty percent of the phosphorus that flows into Lake Kojima comes from farmland.
Therefore, we need to reduce the outflow of phosphorus from farmland.
Compared with standard fertilizers (with an N-P-K ratio of 1-1-1), low P-K fertilizers
contain less phosphoric acid and potassium. If farm soil contains sufficient phosphoric
acid and potassium, effective use of a low P-K fertilizer alleviates excessive dosage of
phosphoric acid and potassium, therefore enabling eco-friendly agriculture without
affecting the yield.