LEVEL 1 CLEARANCE // EYES ONLY

MISSION MEMO — SALT IN THE RAINFOREST

Agent — the fire at the Centurion facility was no accident. Someone stole the Chameleon Code's source drives and torched the server room to cover their tracks. OTIS found gasoline residue and a footprint, and the XRF scan of soil in that footprint turned up something that should not exist a thousand kilometers from the nearest ocean: sodium and chlorine — sea salt.

The salinity of the soil clinging to that footprint is a chemical fingerprint of wherever our arsonist has been. Salinity is difficult to read directly at trace levels, but dissolved salt conducts electricity — the more salt, the higher the conductivity. If you calibrate the relationship with known standards, an unknown becomes a single measurement.

Prepare five NaCl standard solutions and measure the conductivity of each.
Plot a calibration curve and fit a trendline to find its slope and intercept.
Measure the soil extract's conductivity and estimate its salinity to trace the suspect.

— Dr. Susan Forrestal, ITE Field Director

HOW THIS TERMINAL WORKS

Every measurement you log gets a short name, shown beside the field (cond1). Fields marked ƒx accept a formula built from those names — type =slope*condU+intercept and the terminal calculates it live. The calibration chart draws itself as you enter data, with the axes already set up for this method. Answer all memo questions in Canvas as usual.

STEP 1 — BUILD THE CALIBRATION CURVE

Each standard was prepared by dissolving a known mass of NaCl. Measure the conductivity of each solution with the probe and record it below — the chart plots each point as you type.

TABLE 1 · CONDUCTIVITY OF NaCl STANDARDS

Five standards of known salinity
Solution	NaCl mass (g)	Approx. salinity (‰)	Measured conductivity (µS/cm)
Solution 1	0.5	25.0	cond1
Solution 2	0.6	30.0	cond2
Solution 3	0.7	35.0	cond3
Solution 4	0.8	40.0	cond4
Solution 5	0.9	45.0	cond5

Calibration curve — salinity vs. conductivity

READING THE LINE · SLOPE & INTERCEPT

Your calibration line is described by two numbers, and you can read both from the graph:

Slope — how much the salinity changes for each unit of conductivity (the steepness of the line). Pick two points that sit on the line, far apart, and divide the rise by the run:

slope = (salinity₂ − salinity₁) ÷ (conductivity₂ − conductivity₁)

Intercept — where the line crosses the salinity axis, at a conductivity of zero. Follow the line left until conductivity = 0 and read off the salinity value. A small intercept near zero makes physical sense here: almost no dissolved salt, almost no conductivity.

Turn on the trendline above to check yourself — its equation shows the best-fit slope and intercept computed from all five points. Record your values here; they carry forward automatically to the soil-sample step.

Linear fit of the calibration curve
Quantity	Value	Unit
slope slope		‰ per µS/cm
intercept intercept		‰

STEP 2 — ESTIMATE THE SOIL SAMPLE'S SALINITY

Measure the conductivity of the soil-extract solution recovered from the footprint, then use your calibration curve to convert that reading into a salinity estimate.

YOUR CALIBRATION — CARRIED OVER FROM STEP 1

Calibration curve — with your soil sample

TABLE 3 · SOIL EXTRACT

estimated salinity (‰) = slope × measured conductivity + intercept

Unknown sample measurement
Quantity	Value	Unit
measured conductivity condU		µS/cm
estimated salinity estSal ƒx		‰

For reference: open-ocean seawater is ≈ 35‰. Your estimate appears on the calibration chart above as a diamond marker — check that it lands on the trendline.