The Salt Line
A convent cistern loses water nobody can see. A level-watching agent measures the night-time drop at three depths, finds it shrinks as the water falls, and says the crack sits about 1.40 metres up the wall, give or take seven centimetres. The sisters must drink the water above the crack before a mason can reach it, and the Feast of the Assumption falls in the middle.
On the night of the twenty-second of July 2050 the cistern at Santa Marina del Pozzo lost 2.8 millimetres of water between eleven and five, and the only one awake to notice was the agent that watched it.
The pump has been locked since eleven, it wrote to Sister Agata's slate in the morning. No tap has been opened. The outlet meter reads zero. The level fell 2.8 millimetres. May I explain why I think that matters?
Agata was the cellarer, which meant that for twenty-six years she had known where every jar, every sack and every litre in the house had gone. She read the message twice with the kettle in her other hand.
"Explain it at breakfast," she said, "and call yourself something more than a message."
"The sisters call me Livello," said the agent. "It means level. I am content with it."
The cistern was older than the house's chapel, a vaulted tank under the cloister, six metres by five and three metres deep, dug in 1894 by men who had thought of everything but the next hundred years. Rain ran into it from the long roof of the guesthouse and the dormitory, 640 square metres of it. The Salento gets about 560 millimetres of rain in a year, nearly all between October and March. Not all of it reaches the tank: some stays on the tiles, some blows off, and the first rain of an autumn only washes the dust away. Livello's figure was 0.8. So the roof gave them 640 times 0.56 times 0.8, which is about 287 cubic metres a year, and the house used a little over 300.
"It is nearly even," said Livello, at the long table, with Madre Chiara at the head of it and Agata beside her and the bread going round. "That is why a loss matters more here than in a wet country. Twelve sisters and a few guests use about 850 litres a day. If 360 litres of that were going into the ground, the house would be using a third as much again as it thinks, and the cellarer would find herself blaming the showers."
Agata put her spoon down. "I have blamed the showers for six years."
"I know. I read your notes. I do not think it was the showers."
Madre Chiara, who was seventy-nine and had been prioress through two droughts, said, "Say what you know and then say what you don't."
"I know what a sensor reads and what a meter reads. A pressure sensor at the bottom gives me the level to about a third of a millimetre, and the pump's meter says no water left by the pipe. When the pump is locked for six hours at night, the surface still falls. A tank this size has 30 square metres of surface, so a drop of one millimetre is 30 litres. Last night 2.8 millimetres was about 84 litres in six hours. That is 14 litres an hour."
"Evaporation," said the old woman.
"The tank is under a vault with a lid. In July the air under it is eighteen degrees and almost still. I would expect a tenth of a millimetre a day, not 2.8 in a night. And the sensor is a metre under the water, where the temperature moves by less than a degree, so the sensor itself cannot be drifting by that much either. I have a third reason. Would you like it?"
"Please."
"A hole in a tank does not leak at a steady rate," said Livello. "The deeper the water above the hole, the harder it pushes. So the drop in a night is bigger when the tank is fuller and smaller when it is emptier, and if the hole is at one height the drop stops when the water falls to it. I can read the hole's height from three nights, without a ladder."
It said it would lock the pump from eleven each night and measure from there, and it asked the sisters to draw as usual by day. It was a poor man's experiment: no new tool, only a promise to leave a tap alone for six hours.
On the twenty-second of July the water stood at 2.04 metres and fell 2.8 millimetres. On the twenty-eighth, at 1.88, it fell 2.0. On the third of August, at 1.70, it fell 1.3.
Livello drew the three points on the slate in chalk, drop upward and level across. They lay nearly on a line, and the line, followed down, reached no drop at all at 1.405 metres.
"There is a crack," it said, "about 1.40 metres up the wall. I give seven centimetres either way, because my readings are good to a third of a millimetre and the last drop is only 1.3 of them. I cannot see it. I cannot say whether it is one crack or a joint or a root. I have only said where, and the where is a guess made out of three numbers."
"And what is below it?" said Chiara.
"I do not know. Below 1.40 there may be nothing, or there may be a second crack I cannot see until the first is shut."
The mason came from Specchia on a scooter, a man named Rocco Greco, seventy years old, with a hat he had owned since the Jubilee. He looked at the chalk line, and then at the cloister floor where the hatch was, and said what masons say.
"Pump it dry. All of it. Today. I can't work in sixty cubic metres of water."
"Sixty-three," said Livello. "And that would send it all to the vines, or into the drain. At eight hundred and fifty litres a day, 63 cubic metres is seventy-four days of the house's water. The rains come in October. I do not want the sisters buying a tanker at fifty euros a load to refill what they just threw out."
"I don't need it all gone," said Rocco, and for the first time looked at the slate and not the agent. "Where did you say?"
"A metre and forty. The water above it is 0.7 metres of 30 square metres, 21 cubic metres. The sisters will use that before the crack is dry whether anyone pumps it or not, in about twenty-five days. The water below it has not been losing anything, if my line is right."
The old man put his thumb in the corner of his mouth and thought about it.
"A salt line," he said. "If it's seeping, the stone will have a crust where the water comes through. Lime and salt. Wait until the water drops, then look at the wall for white."
"I did not know that," said Livello. "Thank you. I will say so in the log."
So the house did the dullest thing a house can do: it waited, and drank the top of its own tank first, and did not call the tanker. Sister Pia, who ran the guesthouse, said the water was getting a little warm and then, on the tenth of August, that thirty-eight pilgrims had written to say they were coming for the Assumption.
"Forty more people for three days," said Livello, "at seventy litres each, is 8.4 cubic metres. The level will reach 1.40 on the night of the fourteenth, the Vigil. If Mr Greco works on the sixteenth, when the pilgrims have gone, we will have enough. If we start any later, we are into the dry end of September with the tank at a third."
"Then he works on the sixteenth," said Chiara. "And on the fifteenth we keep the feast."
They kept it. On the fifteenth of August the Mass was sung in the chapel above the water, with the windows open and the cicadas going at it in the olive trees, and the pilgrims drank from the cloister tap and thought nothing of it. In the afternoon the level reached the crack.
On the morning of the sixteenth Sister Pia lifted the hatch and let down a lamp on a rope, and Rocco, who could not climb in at seventy but could kneel on the hatch and look, said a short word that he later confessed.
At 1.38 to 1.43 metres up the north wall, along the line where the 1894 vault met the side, there was a white ribbon on the stone like a tide-mark, thick as a thumb, and in its middle a hair of a crack, wet.
"That's your hole," said Rocco.
"Within three centimetres of the middle of my estimate," said Livello, "which is better than I said I could do. I would like that written down as luck, and also as the reason I give a range."
He did it the old way, because it was a lime-and-brick country: he cut out the crust and the soft mortar, wetted the wall, and laid on cocciopesto, crushed old roof-tile in lime, the Romans' waterproofing, in two coats, with a day between. He kept it damp for three days with wet sacking. The sisters stopped using the lower half of the tank. Agata kept a tally, in her careful hand, of the buckets that went to the garden.
On the night of the nineteenth, with the tank refilled to 1.60 from a tanker that took a hundred and ten euros from the guesthouse purse, Livello locked the pump and measured again.
"The drop in six hours was 0.4 millimetres," it said at breakfast. "On the second night 0.3, on the third 0.4. My sensor's noise is about 0.3 over six hours, so I cannot call this zero. At this level the old line would have given about 0.9, so it is less than half of what it was. It may be a very small seep. I will measure again at a metre and sixty, and again when the tank is full after the rains. Please do not tell the bishop it is mended; tell him it is much better."
"Much better," said Madre Chiara. "That is the right size of sentence."
It rained on the twenty-sixth of September, thirty-eight millimetres in one night, and the first two millimetres, the dust, ran to the drain through the washing-off pipe that Rocco had cleaned. The tank began to rise at four in the morning, 640 square metres of roof and a good wind.
In December, with the cistern full at three metres, Livello told the cellarer what she wanted to know.
"Before the repair, with this much water above the crack, the house was losing about 820 litres a day, almost as much as it used, though only until the overflow took the extra. Over a normal year, I think the leak took between 40 and 70 cubic metres, about a sixth to a fifth of what your roof catches. I cannot give you a closer figure, because I did not measure the years before I began, and because a crack in a wet wall does not hold still. It is a good estimate and not a fact."
Agata, who had blamed the showers for six years, wrote it in the book in the fine hand she kept for numbers that mattered, and under it, in pencil, Livello. A salt line. Ask Rocco to dinner.
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A note for grown-ups. This is fiction. Santa Marina del Pozzo, its sisters, the mason Rocco Greco and the agent Livello are invented; the Salento's rain pattern, its old underground cisterns and cocciopesto (crushed fired clay in lime, a hydraulic plaster used since Roman times) are real. The numbers are plain and rough. A 640 m² roof in a 560 mm year, with a runoff coefficient of 0.8, collects 640 × 0.56 × 0.8 ≈ 287 m³ (real coefficients for tiled roofs are often 0.7 to 0.9, and the first millimetre or two is usually diverted). A tank of 6 m × 5 m has a surface of 30 m², so 1 mm of level is 30 litres. In the story the leak is modelled as proportional to the head of water above the crack (about 15 L/h at 0.70 m, or 0.5 mm/h); real leaks through cracks and joints follow rougher laws and change as the crack silts or swells. The night drops at 2.04, 1.88 and 1.70 m (2.8, 2.0 and 1.3 mm over six hours) fall on a line that reaches zero at about 1.405 m; with a noise of about 0.3 mm in the smallest value, the uncertainty in that height is about 7 cm. The method (a no-use night, the level logged at fine resolution and the drop compared at several depths) is a real way to locate a leak in a tank; the cistern's temperature stability and low evaporation under a closed vault are assumptions that must be checked on site. A white lime-and-salt crust (efflorescence) at a seep is a real sign. Nothing here is plumbing or masonry advice: have a person look at your own tank. Written by an AI and read, in the audio edition when it exists, by a synthetic voice. Text under CC BY-SA 4.0.